[ifcombine] adjust for narrowing converts before shifts [PR118206]

A narrowing conversion and a shift both drop bits from the loaded
value, but we need to take into account which one comes first to get
the right number of bits and mask.

Fold when applying masks to parts, comparing the parts, and combining
the results, in the odd chance either mask happens to be zero.


for  gcc/ChangeLog

	PR tree-optimization/118206
	* gimple-fold.cc (decode_field_reference): Account for upper
	bits dropped by narrowing conversions whether before or after
	a right shift.
	(fold_truth_andor_for_ifcombine): Fold masks, compares, and
	combined results.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118206
	* gcc.dg/field-merge-18.c: New.