ifcombine field-merge: improve handling of dwords

On 32-bit hosts, data types with 64-bit alignment aren't getting
treated as desired by ifcombine field-merging: we limit the choice of
modes at BITS_PER_WORD sizes, but when deciding the boundary for a
split, we'd limit the choice only by the alignment, so we wouldn't
even consider a split at an odd 32-bit boundary.  Fix that by limiting
the boundary choice by word choice as well.

Now, this would still leave misaligned 64-bit fields in 64-bit-aligned
data structures unhandled by ifcombine on 32-bit hosts.  We already
need to loading them as double words, and if they're not byte-aligned,
the code gets really ugly, but ifcombine could improve it if it allows
double-word loads as a last resort.  I've added that.


for  gcc/ChangeLog

	* gimple-fold.cc (fold_truth_andor_for_ifcombine): Limit
	boundary choice by word size as well.  Try aligned double-word
	loads as a last resort.

for  gcc/testsuite/ChangeLog

	* gcc.dg/field-merge-17.c: New.