[InstCombine] fold multiply by signbit-splat to cmp+select

(ashr i32 X, 31) * C --> (X < 0) ? -C : 0
https://alive2.llvm.org/ce/z/G8u9SS

With a constant operand, this is an improvement in IR
and codegen (where it can be converted to a mask op).

Without a constant operand, we would have to negate
the operand, so that is probably better left to the backend.

This is similar but not the same optimization that is requested
in #55618.