x86: Insert extra move for mode size smaller than natural size

x86: Insert extra move for mode size smaller than natural size

When generating a SUBREG from V16QI to V2HF, validate_subreg fails since
V2HF is a floating point vector and its size (4 bytes) is smaller than its
natural size (word size).  Insert an extra move with a QI vector SUBREG of
the same size to avoid validate_subreg failure.

gcc/

	PR target/120036
	* config/i386/i386-features.cc (ix86_get_vector_load_mode):
	Handle 8/4/2 bytes.
	(remove_redundant_vector_load): If the mode size is smaller than
	its natural size, first insert an extra move with a QI vector
	SUBREG of the same size to avoid validate_subreg failure.

gcc/testsuite/

	PR target/120036
	* g++.target/i386/pr120036.C: New test.
	* gcc.target/i386/pr117839-3a.c: Likewise.
	* gcc.target/i386/pr117839-3b.c: Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>