…r when it contains an undef

Implement emscripten_float32x4_select, and prepare the UsedVars map in anticipation
of SIMD coercive calls which will need to know the full type.

Map vector load and store instructions to SIMD.js load and store APIs.

LLVM doesn't have vector constructor operators; instead it uses chains
of insertelements. Teach Emscripten to collect chains and detect when
they form splats and constructor calls so that it can emit the natural
API calls.

Instead of taking the array buffer from the typed array view, just
reference the buffer object directly.

This uses the new single-argument SIMD constructor.