AMDGPU: fix local stack slot allocation bugs

Summary:
The main bug fix here is using the 32-bit encoding of V_ADD_I32 in
materializeFrameBaseRegister and resolveFrameIndex, so that arbitrary
immediates work.

The second part is that we may now require the SegmentWaveByteOffset
even when there are initially no stack objects and VGPR spilling isn't
enabled, for stack slots that are allocated later. This means that some
bits become effectively dead and can be cleaned up.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96602


Tested-by: Kai Wasserbäch <kai@dev.carbon-project.org>

Reviewers: arsenm, tstellarAMD

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D21551

llvm-svn: 275108