Before this change traps always returned a 32 bit word in tests. With this
change traps return either a 32 bit word or a64 bit word, depending on the size
of the actual return value of the test.

Additionally this CL implements the wasm instructions I64SCONVERTF32,
I64UCONVERTF32, I64SCONVERTF64, and I64UCONVERTF64.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1519013003

Cr-Commit-Position: refs/heads/master@{#32800}