Change the canonical induction variable that we insert.

Instead of producing code like this:

Loop:
  X = phi 0, X2
  ...

  X2 = X + 1
  if (X != N-1) goto Loop

We now generate code that looks like this:

Loop:
  X = phi 0, X2
  ...

  X2 = X + 1
  if (X2 != N) goto Loop

This has two big advantages:
  1. The trip count of the loop is now explicit in the code, allowing
     the direct implementation of Loop::getTripCount()
  2. This reduces register pressure in the loop, and allows X and X2 to be
     put into the same register.

As a consequence of the second point, the code we generate for loops went
from:

.LBB2:  # no_exit.1
	...
        mov %EDI, %ESI
        inc %EDI
        cmp %ESI, 2
        mov %ESI, %EDI
        jne .LBB2 # PC rel: no_exit.1

To:

.LBB2:  # no_exit.1
	...
        inc %ESI
        cmp %ESI, 3
        jne .LBB2 # PC rel: no_exit.1

... which has two fewer moves, and uses one less register.

llvm-svn: 12961