[mlir] Fix GPU LaunchFunc conversion to the LLVM dialect

The conversion has been incorrectly using the operands of the original
operation instead of the converted operands provided to the matchAndRewrite
call. This may lead to spurious materializations and generally invalid IR if
the producer of the original operands is deleted in the process of conversion.

Reviewed By: csigg

Differential Revision: https://reviews.llvm.org/D109356