AMDGPU: Fix LCSSA phi lowering in SILowerI1Copies

Summary:
When an LCSSA phi survives through instruction selection, the pass
ends up removing that phi entirely because it is dominated by the
logic that does the lanemask merging.

This then used to trigger an assertion when processing a dependent
phi instruction.

Change-Id: Id4949719f8298062fe476a25718acccc109113b6

Reviewers: llvm-commits

Subscribers: kzhuravl, jvesely, wdng, yaxunl, t-tye, tpr, dstuttard, rtaylor, arsenm

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60999

llvm-svn: 358983