ssa: Fix up maybe_rewrite_mem_ref_base complex type handling [PR116034]

ssa: Fix up maybe_rewrite_mem_ref_base complex type handling [PR116034]

The folding into REALPART_EXPR is correct, used only when the mem_offset
is zero, but for IMAGPART_EXPR it didn't check the exact offset value (just
that it is not 0).
The following patch fixes that by using IMAGPART_EXPR only if the offset
is right and using BITFIELD_REF or whatever else otherwise.

2024-07-23  Jakub Jelinek  <jakub@redhat.com>
	    Andrew Pinski  <quic_apinski@quicinc.com>

	PR tree-optimization/116034
	* tree-ssa.cc (maybe_rewrite_mem_ref_base): Only use IMAGPART_EXPR
	if MEM_REF offset is equal to element type size.

	* gcc.dg/pr116034.c: New test.