x86_64: Integer min/max improvements.

x86_64: Integer min/max improvements.

This patch tweaks the way that min and max are expanded, so that the
semantics of these operations are visible to the early RTL optimization
passes, until split into explicit comparison and conditional move
instructions. The good news is that i386.md already contains all of
the required logic (many thanks to Richard Biener and Uros Bizjak),
but this is currently only enabled to scalar-to-vector (STV) synthesis
of min/max instructions.  This change enables this functionality for
all TARGET_CMOVE architectures for SImode, HImode and DImode.

2020-08-06  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (MAXMIN_IMODE): No longer needed.
	(<maxmin><mode>3):  Support SWI248 and general_operand for
	second operand, when TARGET_CMOVE.
	(<maxmin><mode>3_1 splitter): Optimize comparisons against
	0, 1 and -1 to use "test" instead of "cmp".
	(*<maxmin>di3_doubleword): Likewise, allow general_operand
	and enable on TARGET_CMOVE.
	(peephole2): Convert clearing a register after a flag setting
	instruction into an xor followed by the original flag setter.

gcc/testsuite/ChangeLog
	* gcc.target/i386/minmax-8.c: New test.
	* gcc.target/i386/minmax-9.c: New test.
	* gcc.target/i386/minmax-10.c: New test.
	* gcc.target/i386/minmax-11.c: New test.