RISC-V: Avoid redundant sign-extension for SImode SGE, SGEU, SLE, SLEU

RISC-V: Avoid redundant sign-extension for SImode SGE, SGEU, SLE, SLEU

We produce inefficient code for some synthesized SImode conditional set
operations (i.e. ones that are not directly implemented in hardware) on
RV64.  For example a piece of C code like this:

int
sleu (unsigned int x, unsigned int y)
{
  return x <= y;
}

gets compiled (at `-O2') to this:

sleu:
	sgtu	a0,a0,a1	# 9	[c=4 l=4]  *sgtu_disi
	xori	a0,a0,1		# 10	[c=4 l=4]  *xorsi3_internal/1
	andi	a0,a0,1		# 16	[c=4 l=4]  anddi3/1
	ret			# 25	[c=0 l=4]  simple_return

or (at `-O1') to this:

sleu:
	sgtu	a0,a0,a1	# 9	[c=4 l=4]  *sgtu_disi
	xori	a0,a0,1		# 10	[c=4 l=4]  *xorsi3_internal/1
	sext.w	a0,a0		# 16	[c=4 l=4]  extendsidi2/0
	ret			# 24	[c=0 l=4]  simple_return

This is because the middle end expands a SLEU operation missing from
RISC-V hardware into a sequence of a SImode SGTU operation followed by
an explicit SImode XORI operation with immediate 1.  And while the SGTU
machine instruction (alias SLTU with the input operands swapped) gives a
properly sign-extended 32-bit result which is valid both as a SImode or
a DImode operand the middle end does not see that through a SImode XORI
operation, because we tell the middle end that the RISC-V target (unlike
MIPS) may hold values in DImode integer registers that are valid for
SImode operations even if not properly sign-extended.

However the RISC-V psABI requires that 32-bit function arguments and
results passed in 64-bit integer registers be properly sign-extended, so
this is explicitly done at the conclusion of the function.

Fix this by making the backend use a sequence of a DImode SGTU operation
followed by a SImode SEQZ operation instead.  The latter operation is
known by the middle end to produce a properly sign-extended 32-bit
result and therefore combine gets rid of the sign-extension operation
that follows and actually folds it into the very same XORI machine
operation resulting in:

sleu:
	sgtu	a0,a0,a1	# 9	[c=4 l=4]  *sgtu_didi
	xori	a0,a0,1		# 16	[c=4 l=4]  xordi3/1
	ret			# 25	[c=0 l=4]  simple_return

instead (although the SEQZ alias SLTIU against immediate 1 machine
instruction would equally do and is actually retained at `-O0').  This
is handled analogously for the remaining synthesized operations of this
kind, i.e. `SLE', `SGEU', and `SGE'.

	gcc/
	* config/riscv/riscv.cc (riscv_emit_int_order_test): Use EQ 0
	rather that XOR 1 for LE and LEU operations.

	gcc/testsuite/
	* gcc.target/riscv/sge.c: New test.
	* gcc.target/riscv/sgeu.c: New test.
	* gcc.target/riscv/sle.c: New test.
	* gcc.target/riscv/sleu.c: New test.