[AMDGPU] Move LowerSwitch pass to CodeGenPrepare.

It is possible that LowerSwitch pass leaves certain blocks
unreachable from the entry. If not removed, these dead blocks
can cause undefined behavior in the subsequent passes.
It caused a crash in the AMDGPU backend after the instruction
selection when a PHI node has its incoming values coming from
these unreachable blocks.

In the AMDGPU pass flow, the last invocation of UnreachableBlockElim
precedes where LowerSwitch is currently placed and eventually
missed out on the opportunity to get these blocks eliminated.
This patch ensures that LowerSwitch pass get inserted earlier
to make use of the existing unreachable block elimination pass.

Reviewed By: sameerds, arsenm

Differential Revision: https://reviews.llvm.org/D83584