This should fix the bug seen with some registers not being allocated

This should fix the bug seen with some registers not being allocated
correctly: skipping instructions by incorrectly incrementing the pointer.

Also adds support for building a reg-to-regclass map, and splits the function
for saving register to stack into two, one suitable for virtual registers
(which also assigns it a physical register) and one for simply storing back
physical registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4898 91177308-0d34-0410-b5e6-96231b3b80d8