[PPC]: Peephole optimize small accesss to aligned globals.

Access to aligned globals gives us a chance to peephole optimize nonzero
offsets. If a struct is 4 byte aligned, then accesses to bytes 0-3 won't
overflow the available displacement. For example:
        addis 3, 2, b4v@toc@ha
        addi 4, 3, b4v@toc@l
        lbz 5, b4v@toc@l(3) ; This is the result of the current peephole
        lbz 6, 1(4)         ; optimizer
        lbz 7, 2(4)
        lbz 8, 3(4)
If b4v is 4-byte aligned, we can skip using register 4 because we know
that b4v@toc@l+{1,2,3} won't overflow 32K, and instead generate:
        addis 3, 2, b4v@toc@ha
        lbz 4, b4v@toc@l(3)
        lbz 5, b4v@toc@l+1(3)
        lbz 6, b4v@toc@l+2(3)
        lbz 7, b4v@toc@l+3(3)
Saving a register and an addition.
Larger alignments allow larger structures/arrays to be optimized.

llvm-svn: 255319