forwprop: Pun with integer type if needed [PR123799].

forwprop: Pun with integer type if needed [PR123799].

We cannot directly build vectors from BitInts, which this patch
circumvents by punning  the conversion element type with an integer
type.  This results in two separate conversions at the gimple level.
Those don't appear to cause worse final code, though.  It is possible to
merge those two conversions at the construction site but from what I can
tell would involve more changes than necessary now, so I refrained from
it.

Before this patch we would check tree_nop_conversion_p for e.g.
BitInt _12 = BIT_FIELD_REF (vector unsigned int).  This is a
"nop conversion" but the implicit assumption is that we can build
a vector type from the lhs type that can be nop-converted back to
the original type.  This is not true for BitInt.

Bootstrapped and regtested on x86, power10, and aarch64.
Regtested on riscv64.

	PR tree-optimization/123799

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (simplify_vector_constructor): Pun
	conversion element type with integer type.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/pr123799.c: New test.