[ARM] Thumb2: favor R4-R7 over R12/LR in allocation order when opt for minsize

For Thumb2, we prefer low regs (costPerUse = 0) to allow narrow
encoding. However, current allocation order is like:
  R0-R3, R12, LR, R4-R11

As a result, a lot of instructs that use R12/LR will be wide instrs.

This patch changes the allocation order to:
  R0-R7, R12, LR, R8-R11
for thumb2 and -Osize.

In most cases, there is no extra push/pop instrs as they will be folded
into existing ones. There might be slight performance impact due to more
stack usage, so we only enable it when opt for min size.

https://reviews.llvm.org/D30324

llvm-svn: 365014