Adds I32UConvertF32, I32SConvertF64, and I32UConvertF64 instructions.

Refactors code to use templates where appropriate, and to use
previously committed template function is_inbounds() when appropriate
in tests.

Bug: v8:7226
Change-Id: I2701e5fd0b21cefa1f285677f20616cfde29ab0d
Reviewed-on: https://chromium-review.googlesource.com/862609
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50632}