i386: Fix TARGET_USE_VECTOR_FP_CONVERTS SF->DF float_extend splitter [PR113133]

The post-reload splitter currently allows xmm16+ registers with TARGET_EVEX512.
The splitter changes SFmode of the output operand to V4SFmode, but the vector
mode is currently unsupported in xmm16+ without TARGET_AVX512VL. lowpart_subreg
returns NULL_RTX in this case and the compilation fails with invalid RTX.

The patch removes support for x/ymm16+ registers with TARGET_EVEX512.  The
support should be restored once ix86_hard_regno_mode_ok is fixed to allow
16-byte modes in x/ymm16+ with TARGET_EVEX512.

	PR target/113133

gcc/ChangeLog:

	* config/i386/i386.md
	(TARGET_USE_VECTOR_FP_CONVERTS SF->DF float_extend splitter):
	Do not handle xmm16+ with TARGET_EVEX512.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr113133-1.c: New test.
	* gcc.target/i386/pr113133-2.c: New test.