In LLVM3, this conversion was subtly broken. We're now on LLVM7, where this should be
fixed.

Mostly a revert of
https://swiftshader-review.googlesource.com/c/SwiftShader/+/7354, but
also includes a subzero implementation

Bug: b/31816482
Change-Id: I154a221b44b985a35bcc5e15923a0deb9b51c711
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39148Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>