[SelectionDAGISel] small cleanup to INLINEASM_BR selection. NFC

Summary:
This code was throwing away the opcode for a boolean, which was then
reconstructing the opcode from that boolean.  Just pass the opcode, and
forget the boolean.

Reviewers: srhines

Reviewed By: srhines

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77100