[AMDGPU] Remove the assertion for MUBUF instruction with voffset

Currently, there is an assertion that limits the MUBUF instruction
with voffset used for a VGPR spill inside kernel functions when
the frame pointer is really used. It seemed more like an unwanted
limitation and hence removing the assertion.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D137892