[mlir][spirv] Don't return value when cannot fold spirv.bitcast

Returing a value would make the canonicalization infrastructure
think that folding succeeded so the pattern will be tried again
when invoked via, e.g., `applyPatternsAndFoldGreedily` and
eventually fail due to not converging after 10 times by default.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D137598