[WebAssembly] Implement v128.select

Although the SIMD spec proposal does not specifically include a
select instruction, the select instruction in MVP WebAssembly is
polymorphic over the selected types, so it is able to work on v128
values when they are enabled. This patch introduces a new variant of
the select instruction for each legal vector type. Additional ISel
patterns are adapted from the SELECT_I32 and SELECT_I64 patterns.

Depends on D83736.

Differential Revision: https://reviews.llvm.org/D83737