RISC-V: Add blocker for gather/scatter auto-vectorization

RISC-V: Add blocker for gather/scatter auto-vectorization

This patch fixes ICE exposed on full coverage testing:

                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=dynamic ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)
                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=dynamic --param=riscv-autovec-preference=fixed-vlmax ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)
                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=m4 ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)
                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=m4 --param=riscv-autovec-preference=fixed-vlmax ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)
                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=m8 ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)
                                === g++: Unexpected fails for rv64gc_zve32f_zvfh_zfh lp64d medlow --param=riscv-autovec-lmul=m8 --param=riscv-autovec-preference=fixed-vlmax ===
FAIL: g++.dg/pr106219.C  -std=gnu++14 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++17 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++20 (internal compiler error: in require, at machmode.h:313)
FAIL: g++.dg/pr106219.C  -std=gnu++98 (internal compiler error: in require, at machmode.h:313)

The rootcause is we can't extend RVVM4SImode into RVVM8DImode on zve32f.
Add a blocker of it to disable such auto-vectorization in this situation.

gcc/ChangeLog:

	* config/riscv/autovec.md: Add blocker.
	* config/riscv/riscv-protos.h (gather_scatter_valid_offset_p): New function.
	* config/riscv/riscv-v.cc (gather_scatter_valid_offset_p): Ditto.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/autovec/bug-2.C: New test.