[PowerPC] Guard against illegal selection of add for TargetConstant operands

r208640 was reverted because it caused a self-hosting failure on ppc64. The
underlying cause was the formation of ISD::ADD nodes with ISD::TargetConstant
operands. Because we have no patterns for 'add' taking 'timm' nodes, these are
selected as r+r add instructions (which is a miscompile). Guard against this
kind of behavior in the future by making the backend crash should this occur
(instead of silently generating invalid output).
Revert "Revert '[DAGCombiner] Split up an indexed load if only the base pointer value is live'"

I reverted r208640 in r209747 because r208640 broke self-hosting on PPC64. The
underlying cause of the failure is that pre-inc loads with increments
represented by ISD::TargetConstants were being transformed into ISD:::ADDs with
ISD::TargetConstant operands. PPC doesn't have a pattern for those, and so they
were selected as invalid r+r adds.

This recommits r208640, rebased and with an exclusion for ISD::TargetConstant
increments. This behavior seems correct, although in the future we might want
to ask the target to split out the indexing that uses ISD::TargetConstants.

Unfortunately, I don't yet have small test case where the relevant invalid
'add' instruction is not itself dead (and thus eliminated by
DeadMachineInstructionElim -- sometimes bugpoint is too good at removing things)

Original commit message (by Adam Nemet):

Right now the load may not get DCE'd because of the side-effect of updating
the base pointer.

This can happen if we lower a read-modify-write of an illegal larger type
(e.g. i48) such that the modification only affects one of the subparts (the
lower i32 part but not the higher i16 part).  See the testcase.

In order to spot the dead load we need to revisit it when SimplifyDemandedBits
decided that the value of the load is masked off.  This is the
CommitTargetLoweringOpt piece.

I checked compile time with ARM64 by sending SPEC bitcode files through llc.
No measurable change.

Fixes <rdar://problem/16031651>