middle-end/101530 - fix shufflevector lowering

middle-end/101530 - fix shufflevector lowering

This makes __builtin_shufflevector lowering force the result
of the BIT_FIELD_REF lowpart operation to a temporary as to
fulfil the IL verifier constraint that BIT_FIELD_REFs should
be always in outermost handled component position.  Trying to
enforce this during gimplification isn't as straight-forward
as here where we know we're dealing with an rvalue.

FAIL: c-c++-common/torture/builtin-shufflevector-1.c   -O0  execution test

2022-01-05  Richard Biener  <rguenther@suse.de>

	PR middle-end/101530
gcc/c-family/
	* c-common.c (c_build_shufflevector): Wrap the BIT_FIELD_REF
	in a TARGET_EXPR to force a temporary.

gcc/testsuite/
	* c-c++-common/builtin-shufflevector-3.c: New testcase.