Fix some uses of getSubRegisters() to use getSubReg() instead.

Fix some uses of getSubRegisters() to use getSubReg() instead.

It is better to address sub-registers directly by name instead of
relying on their position in the sub-register list.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157703 91177308-0d34-0410-b5e6-96231b3b80d8