Treat boolean vector elements as 0/-1 [PR115406]HEADtrunkmaster

Previously we built vector boolean constants using 1 for true
elements and 0 for false elements.  This matches the predicates
produced by SVE's PTRUE instruction, but leads to a miscompilation
on AVX, where all bits of a boolean element should be set.

One option for RTL would be to make this target-configurable.
But that isn't really possible at the tree level, where vectors
should work in a more target-independent way.  (There is currently
no way to create a "generic" packed boolean vector, but never say
never :))  And, if we were going to pick a generic behaviour,
it would make sense to use 0/-1 rather than 0/1, for consistency
with integer vectors.

Both behaviours should work with SVE on read, since SVE ignores
the upper bits in each predicate element.  And the choice shouldn't
make much difference for RTL, since all SVE predicate modes are
expressed as vectors of BI, rather than of multi-bit booleans.

I suspect there might be some fallout from this change on SVE.
But I think we should at least give it a go, and see whether any
fallout provides a strong counterargument against the approach.

gcc/
	PR middle-end/115406
	* fold-const.cc (native_encode_vector_part): For vector booleans,
	check whether an element is nonzero and, if so, set all of the
	correspending bits in the target image.
	* simplify-rtx.cc (native_encode_rtx): Likewise.

gcc/testsuite/
	PR middle-end/115406
	* gcc.dg/torture/pr115406.c: New test.

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