[mlir][spirv] Timely fail type conversion

Authored by antiagainst on Apr 7 2021, 11:52 AM.


[mlir][spirv] Timely fail type conversion

Per the TypeConverter API contract, returning llvm:None means
other conversion rules should be tried. But we only have one
rule per input type. So there is no need to try others and we can
just directly fail, which should return nullptr. This avoids
unnecessary checks.

Differential Revision: https://reviews.llvm.org/D100058