[AMDGPU] Fix Vreg_1 PHI lowering in SILowerI1Copies.

There is a minor flaw in the implementation of function lowerPhis.
This function replaces values of regclass Vreg_1 (boolean values)
involved in PHIs into an SGPR. Currently it iterates over the MBBs
and performs an inplace lowering of PHIs and fails to lower any
incoming value that itself is another PHI of Vreg_1 regclass.
The failure occurs only when the MBB where the incoming PHI value
belongs is not visited/lowered yet.

To fix this problem, collect all Vreg_1 PHIs upfront and then
perform the lowering.

Differential Revision: https://reviews.llvm.org/D69182