[mlir] Do not print back 0 alignment in LLVM dialect 'alloca' op

The alignment attribute in the 'alloca' op treats the '0' value as 'unset'.
When parsing the custom form of the 'alloca' op, ignore the alignment attribute
with if its value is '0' instead of actually creating it and producing a
slightly different textually yet equivalent semantically form in the output.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D90179