[AArch64] Add more efficient vector bitcast for AArch64

Adds a DAG combine checks for vector comparisons followed by a bitcast to a
scalar value. Previously, this resulted in an expand.  Now, this is done with a
constant number of instructions that take one bit per vector value (via an AND
mask) and perfom a horizontal add to get a single value. This is especially
useful for Clang's __builtin_convertvector() to a bool vector.

Issue: https://github.com/llvm/llvm-project/issues/59829

Differential Revision: https://reviews.llvm.org/D145301