[llvm] [AMDGPU] Fix mul combine for MUL24 (PR #79110)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

"mul24 x, (add y, 1) -> add (mul24 x, y), x" does not seem safe at all. You know that y+1 is a 24-bit value, but that does not guarantee that y is a 24-bit value.

https://github.com/llvm/llvm-project/pull/79110