tree-optimization/110838 - vectorization of widened shifts

The following makes sure to limit the shift operand when vectorizing
(short)((int)x >> 31) via (short)x >> 31 as the out of bounds shift
operand otherwise invokes undefined behavior.  When we determine
whether we can demote the operand we know we at most shift in the
sign bit so we can adjust the shift amount.

Note this has the possibility of un-CSEing common shift operands
as there's no good way to share pattern stmts between patterns.
We'd have to separately pattern recognize the definition.

	PR tree-optimization/110838
	* tree-vect-patterns.cc (vect_recog_over_widening_pattern):
	Adjust the shift operand of RSHIFT_EXPRs.

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

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