rs6000: Support more short/char to float conversion

For some cases that when we load unsigned char/short values from
the appropriate unsigned char/short memories and convert them to
double/single precision floating point value, there would be
implicit conversions to int first.  It makes GCC not leverage the
P9 instructions lxsibzx/lxsihzx.  This patch is to add the related
define_insn_and_split to support this kind of scenario.

Bootstrapped/regtested on powerpc64le-linux-gnu P9 and
powerpc64-linux-gnu P8.

gcc/ChangeLog:

	* config/rs6000/rs6000.md
	(floatsi<SFDF:mode>2_lfiwax_<QHI:mode>_mem_zext): New
	define_insn_and_split.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/p9-fpcvt-3.c: New test.