[AMDGPU] Make sure to fix implicit operands on insertBranch

Summary:
Without fixImplicitOperands we may end up creating default implicit operands
that are the wrong wave size

Includes simple test that provokes insertBranch in the correct way to expose the
issue being fixed.

Change-Id: I92bdcdee9fcb7b4d91529b84e76a48ac8218483e

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82459