[DAG] PromoteIntRes_BUILD_VECTOR - extend constant boolean vectors according...

[DAG] PromoteIntRes_BUILD_VECTOR - extend constant boolean vectors according to target BooleanContents

PromoteIntRes_BUILD_VECTOR currently always ANY_EXTENDs build vector operands, but if this is a constant boolean vector we're losing the useful ability to keep the vector matching the BooleanContents mode used by the target.

This patch extends constant boolean vectors according to target BooleanContents, allowing a number of additional all-bits folds (notable XOR -> NOT conversions) to occur.

Differential Revision: https://reviews.llvm.org/D129641