[DAG] FoldConstantArithmetic - add initial support for undef elements in...

[DAG] FoldConstantArithmetic - add initial support for undef elements in bitcasted binop constant folding

FoldConstantArithmetic can fold constant vectors hidden behind bitcasts (e.g. vXi64 -> v2Xi32 on 32-bit platforms), but currently bails if either vector contains undef elements. These undefs can often occur due to SimplifyDemandedBits/VectorElts calls recognising that the upper bits are often unnecessary (e.g. funnel-shift/rotate implicit-modulo and AND masks).

This patch adds a basic 'FoldValueWithUndef' handler that will attempt to constant fold if one or both of the ops are undef - so far this just handles the AND and MUL cases where we always fold to zero.

The RISCV codegen increase is interesting - it looks like the BUILD_VECTOR lowering was loading a constant pool entry but now (with all elements defined constant) it can materialize the constant instead?

Differential Revision: https://reviews.llvm.org/D130839