Start converting NEON load/stores to use pseudo instructions, beginning here

Start converting NEON load/stores to use pseudo instructions, beginning here
with the VST4 instructions.  Until after register allocation, we want to
represent sets of adjacent registers by a single super-register.  These
VST4 pseudo instructions have a single QQ or QQQQ source register operand.
They get expanded to the real VST4 instructions with 4 separate D register
operands.  Once this conversion is complete, we'll be able to remove the
NEONPreAllocPass and avoid some fragile and hacky code elsewhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112108 91177308-0d34-0410-b5e6-96231b3b80d8