[AArch64] Fold Mul(And(Srl(X, 15), 0x10001), 0xffff) to CMLTz

This folds a v4i32 Mul(And(Srl(X, 15), 0x10001), 0xffff) into a v8i16
CMLTz instruction. The Srl and And extract the top bit (whether the
input is negative) and the Mul sets all values in the i16 half to all
1/0 depending on if that top bit was set. This is equivalent to a v8i16
CMLTz instruction. The same applies to other sizes with equivalent
constants.

Differential Revision: https://reviews.llvm.org/D130874