[MIRPraser] Improve error checking for typed immediate operands

Summary:
This improves error checks for typed immediate operands introduced in
D45948 (rL331586), and removes a code block copied by mistake.

Reviewers: rtereshin

Subscribers: dschuff, sbc100, jgravelle-google, llvm-commits

Differential Revision: https://reviews.llvm.org/D46491

llvm-svn: 331600