ifcombine field merge: do not follow a second conversion [PR118046]

ifcombine field merge: do not follow a second conversion [PR118046]

The testcase shows that conversions that would impact negatively the
ifcombine field merging implementation won't always have been
optimized out by the time we reach ifcombine.

There's probably room to support multiple conversions with extra
logic, but this workaround should avoid codegen errors until that
logic is figured out.

for  gcc/ChangeLog

	PR tree-optimization/118046
	* gimple-fold.cc (decode_field_reference): Don't follow more
	than one conversion.

for  gcc/testsuite/ChangeLog

	PR tree-optimization/118046
	* gcc.dg/field-merge-14.c: New.