AMDGPU: Fix an assertion in SIOptimizeVGPRLiveRange

As the comment notes, the shader results in an INSERT_SUBREG with
"undef" (dead) operand in the Endif block. The same can happen with
REG_SEQUENCE. The register is considered dead from a liveness
analysis perspective. The correct thing to do seems to be nothing:
we keep the undef use of the register, the register allocator should
still be able to take the liveness into account correctly.

Differential Revision: https://reviews.llvm.org/D149161