middle-end/110237 - wrong MEM_ATTRs for partial loads/stores

middle-end/110237 - wrong MEM_ATTRs for partial loads/stores

The following addresses a miscompilation by RTL scheduling related
to the representation of masked stores.  For that we have

(insn 38 35 39 3 (set (mem:V16SI (plus:DI (reg:DI 40 r12 [orig:90 _22 ] [90])
                (const:DI (plus:DI (symbol_ref:DI ("b") [flags 0x2]  <var_decl 0x7ffff6e28d80 b>)
                        (const_int -4 [0xfffffffffffffffc])))) [1 MEM <vector(16) int> [(int *)vectp_b.12_28]+0 S64 A32])
        (vec_merge:V16SI (reg:V16SI 20 xmm0 [118])
            (mem:V16SI (plus:DI (reg:DI 40 r12 [orig:90 _22 ] [90])
                    (const:DI (plus:DI (symbol_ref:DI ("b") [flags 0x2]  <var_decl 0x7ffff6e28d80 b>)
                            (const_int -4 [0xfffffffffffffffc])))) [1 MEM <vector(16) int> [(int *)vectp_b.12_28]+0 S64 A32])

and specifically the memory attributes

  [1 MEM <vector(16) int> [(int *)vectp_b.12_28]+0 S64 A32]

are problematic.  They tell us the instruction stores and reads a full
vector which it if course does not.  There isn't any good MEM_EXPR
we can use here (we lack a way to just specify a pointer and restrict
info for example), and since the MEMs have a vector mode it's
difficult in general as passes do not need to look at the memory
attributes at all.

The easiest way to avoid running into the alias analysis problem is
to scrap the MEM_EXPR when we expand the internal functions for
partial loads/stores.  That avoids the disambiguation we run into
which is realizing that we store to an object of less size as
the size of the mode we appear to store.

After the patch we see just

  [1  S64 A32]

so we preserve the alias set, the alignment and the size (the size
is redundant if the MEM insn't BLKmode).  That's still not good
in case the RTL alias oracle would implement the same
disambiguation but it fends off the gimple one.

This fixes gcc.dg/torture/pr58955-2.c when built with AVX512
and --param=vect-partial-vector-usage=1.

	PR middle-end/110237
	* internal-fn.cc (expand_partial_load_optab_fn): Clear
	MEM_EXPR and MEM_OFFSET.
	(expand_partial_store_optab_fn): Likewise.