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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246354 91177308-0d34-0410-b5e6-96231b3b80d8