Recommit "[AArch64] Select SMULL for zero extended vectors when top bit is zero"

This is a recommit of f9e03907
The previous commit failed to handle cases where the zero extended operand is an extended `BUILD_VECTOR`.
We don't replace zext with a sext operand to select smull if any operand is `BUILD_VECTOR`

Original commit message:

we can safely replace a `zext` instruction with `sext` if the top bit is zero. This is useful because we can select `smull` when both operands are sign extended.

Reviewed By: fhahn, dmgreen

Differential Revision: https://reviews.llvm.org/D134711