[AMDGPU] Fixed SIInstrInfo::getOpSize to handle subregs

Currently it returns incorrect operand size for a target independet
node such as COPY if operand is a register with subreg. Instead of
correct subreg size it returns a size of the whole superreg.

Differential Revision: https://reviews.llvm.org/D52736

llvm-svn: 343508