[PR100106] Reject unaligned subregs when strict alignment is required

[PR100106] Reject unaligned subregs when strict alignment is required

The testcase for pr100106, compiled with optimization for 32-bit
powerpc -mcpu=604 with -mstrict-align expands the initialization of a
union from a float _Complex value into a load from an SCmode
constant pool entry, aligned to 4 bytes, into a DImode pseudo,
requiring 8-byte alignment.

The patch that introduced the testcase modified simplify_subreg to
avoid changing the MEM to outermode, but simplify_gen_subreg still
creates a SUBREG or a MEM that would require stricter alignment than
MEM's, and lra_constraints appears to get confused by that, repeatedly
creating unsatisfiable reloads for the SUBREG until it exceeds the
insn count.

Avoiding the unaligned SUBREG, expand splits the DImode dest into
SUBREGs and loads each SImode word of the constant pool with the
proper alignment.

for  gcc/ChangeLog

	PR target/100106
	* emit-rtl.cc (validate_subreg): Reject a SUBREG of a MEM that
	requires stricter alignment than MEM's.

for  gcc/testsuite/ChangeLog

	PR target/100106
	* gcc.target/powerpc/pr100106-sa.c: New.