[PowerPC] Constrain base register in PPCRegisterInfo::resolveFrameIndex

I've run into a bug where current LLVM at -O0 (with fast-isel)
generated invalid code like:

        ld 0, 20936(1)                  # 8-byte Folded Reload
        stw 12, 10348(0)
        stw 12, 10344(0)

The underlying vreg had been introduced as base register by the
Local Stack Slot Allocation pass.  That register was constrained
to G8RC by PPCRegisterInfo::materializeFrameBaseRegister to match
the ADDI instruction used to set it, but it was *not* constrained
to G8RC_NOX0 to fit the *use* of the register in an address.

That should have happened in PPCRegisterInfo::resolveFrameIndex.
This patch adds an appropriate constrainRegClass call.

Reviewed by Hal Finkel.