[DAG] FoldConstantArithmetic - fold bitlogic(bitcast(x),bitcast(y)) -> bitcast(bitlogic(x,y))

To constant fold bitwise logic ops where we've legalized constant build vectors to a different type (e.g. v2i64 -> v4i32), this patch adds a basic ability to peek through the bitcasts and perform the constant fold on the inner operands.

The MVE predicate v2i64 regressions will be addressed by future support for basic v2i64 type support.

One of the yak shaving fixes for D113192....

Differential Revision: https://reviews.llvm.org/D113202