AMDGPU/SI: Fix some invaild assumptions when folding 64-bit immediates

Summary:
We were assuming tha if the use operand had a sub-register that
the immediate was 64-bits, but this was breaking the case of
folding a 64-bit immediate into another 64-bit instruction.

Reviewers: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D12255

llvm-svn: 246354