[WASM] Fix legalizer for LowerBUILD_VECTOR.

Constants in BUILD_VECTOR may be down cast into a smaller value that fits LaneBits, i.e., the bit width of elements in the vector.
This cast didn't consider 2^N where it would be cast into -2^N, which still doesn't fit into LaneBits after casting.
This will cause an assertion in later legalization.

2^N should be cast into 0, and this patch reflects such behavior.
This patch also includes a test to reflect the fix.
This patch fixes [issue 61780](https://github.com/llvm/llvm-project/issues/61780)

Related patch: https://reviews.llvm.org/D108669

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D147208