tree-optimization/110838 - vectorization of widened right shifts

The following fixes a problem with my last attempt of avoiding
out-of-bound shift values for vectorized right shifts of widened
operands.  Instead of truncating the shift amount with a bitwise
and we actually need to saturate it to the target precision.

The following does that and adds test coverage for the constant
and invariant but variable case that would previously have failed.

	PR tree-optimization/110838
	* tree-vect-patterns.cc (vect_recog_over_widening_pattern):
	Fix right-shift value sanitizing.  Properly emit external
	def mangling in the preheader rather than in the pattern
	def sequence where it will fail vectorizing.

	* gcc.dg/vect/pr110838.c: New testcase.