Bug 257842

rdar://108643371

B3 Select lowers to Air using one of several conditional moves, like MoveConditionally64 or MoveDoubleConditionallyFloat. These in turn generate native conditional move instructions, such as fcsel to select between floating-point registers on ARM. However, we don't implement conditional moves for vectors, so when we compile a WASM select instruction with vector operands, B3 ends up treating them like doubles, which ends up zeroing the upper half of the vector register.