[AMDGPU] Trim trailing undefs from the end of image and buffer store

Remove undef values from the end of the vector operand in image and
buffer store instructions.
Also instead of call to computeKnownFPClass, use only findScalarElement.

Continuation of: 88421ea9 Trim zero components from buffer and image stores

Differential Revision: https://reviews.llvm.org/D152440