- OpSConvert, OpUConvert, OpFConvert have no valid use, as we only have
  one value width (32 bits) and these instructions perform width
  conversions only.
- OpConvertFToU, OpConvertFToS, OpConvertUToF, OpConvertSToF implemented
- OpBitcast implemented. Note that the spec looks scary wrt pointer
  types in OpBitcast, but 2.16.1 Universal Validation Rules disallows
  this use in the Logical addressing model.

Bug: b/126952020
Change-Id: I6c1c95d5ad4e19177e40ead7713bf63ffe16c679
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26010Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>