tree-optimization/116024 - simplify C1-X cmp C2 for UB-on-overflow types

tree-optimization/116024 - simplify C1-X cmp C2 for UB-on-overflow types

Implement a match.pd pattern for C1 - X cmp C2, where C1 and C2 are
integer constants and X is of a UB-on-overflow type.  The pattern is
simplified to X rcmp C1 - C2 by moving X and C2 to the other side of the
comparison (with opposite signs).  If C1 - C2 happens to overflow,
replace the whole expression with either a constant 0 or a constant 1
node, depending on the comparison operator and the sign of the overflow.

This transformation allows to occasionally save load-immediate /
subtraction instructions, e.g. the following statement:

10 - (int) x <= 9;

now compiles to

sgt     a0,a0,zero

instead of

li      a5,10
sub     a0,a5,a0
slti    a0,a0,10

on 32-bit RISC-V.

Additional examples can be found in the newly added test file. This
patch has been bootstrapped and regtested on aarch64, x86_64, and
i386, and additionally regtested on riscv32.  Existing tests were
adjusted where necessary.

gcc/ChangeLog:

	PR tree-optimization/116024
	* match.pd: New transformation around integer comparison.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr116024.c: New test.
	* gcc.dg/pr67089-6.c: Adjust.