tree-optimization/122577 - missed vectorization of conversion from bool

tree-optimization/122577 - missed vectorization of conversion from bool

We are currently overly restrictive with rejecting conversions from
bit-precision entities to mode precision ones.  Similar to RTL expansion
we can focus on non-bit operations producing bit-precision results
which we currently do not properly handle by masking.  Such checks
should be already present.  The following relaxes vectorizable_conversion.

Actual bitfield accesses are catched and rejected by vectorizer dataref
analysis and converted during if-conversion into mode-size accesses
with appropriate sign- or zero-extension.

	PR tree-optimization/122577
	* tree-vect-stmts.cc (vectorizable_conversion): Allow conversions
	from non-mode-precision types.

	* gcc.dg/vect/vect-bool-3.c: New testcase.