[PATCH v2] RISC-V: Improve code generation for select of consecutive constants

Based on the valuable feedback I received, I decided to implement the patch
in the RTL pipeline. Since a similar optimization already exists in
simplify_binary_operation_1, I chose to generalize my original approach
and place it directly below that code.

The expression (X xor C1) + C2 is simplified to X xor (C1 xor C2) under
the conditions described in the patch. This is a more general optimization,
but it still applies to the RISC-V case, which was my initial goal:

long f1(long x, long y) {
    return (x > y) ? 2 : 3;
}

Before the patch, the generated assembly is:

f1(long, long):
        sgt     a0,a0,a1
        xori    a0,a0,1
        addi    a0,a0,2
        ret

After the patch, the generated assembly is:

f1(long, long):
        sgt     a0,a0,a1
        xori    a0,a0,3
        ret

The patch optimizes cases like x LT/GT y ? 2 : 3 (and x GE/LE y ? 3 : 2),
as initially intended. Since this optimization is more general, I noticed
it also optimizes cases like x < CONST ? 3 : 2 when CONST < 0. I’ve added
tests for these cases as well.

A bit of logic behind the patch: The equality A + B == A ^ B + 2 * (A & B)
always holds true. This can be simplified to A ^ B if 2 * (A & B) == 0.
In our case, we have A == X ^ C1, B == C2 and X is either 0 or 1.

	PR target/108038

gcc/ChangeLog:

	* simplify-rtx.cc (simplify_context::simplify_binary_operation_1): New
	simplification.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/slt-1.c: New test.

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