RISC-V: Support highpart overlap for floating-point widen instructions
This patch leverages the approach of vwcvt/vext.vf2 which has been approved.
Their approaches are totally the same.
Tested no regression and committed.
PR target/112431
gcc/ChangeLog:
* config/riscv/vector.md: Add widenning overlap.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/rvv/base/pr112431-10.c: New test.
* gcc.target/riscv/rvv/base/pr112431-11.c: New test.
* gcc.target/riscv/rvv/base/pr112431-12.c: New test.
* gcc.target/riscv/rvv/base/pr112431-13.c: New test.
* gcc.target/riscv/rvv/base/pr112431-14.c: New test.
* gcc.target/riscv/rvv/base/pr112431-15.c: New test.
* gcc.target/riscv/rvv/base/pr112431-7.c: New test.
* gcc.target/riscv/rvv/base/pr112431-8.c: New test.
* gcc.target/riscv/rvv/base/pr112431-9.c: New test.