Recommit "Use ValueOffsetPair to enhance value reuse during SCEV
expansion".

The fix for PR28705 will be committed consecutively.

In D12090, the ExprValueMap was added to reuse existing value during
SCEV expansion. However, const folding and sext/zext distribution can
make the reuse still difficult.

A simplified case is: suppose we know S1 expands to V1 in
ExprValueMap, and
  S1 = S2 + C_a
  S3 = S2 + C_b
where C_a and C_b are different SCEVConstants. Then we'd like to
expand S3 as V1 - C_a + C_b instead of expanding S2 literally. It is
helpful when S2 is a complex SCEV expr and S2 has no entry in
ExprValueMap, which is usually caused by the fact that S3 is
generated from S1 after const folding.

In order to do that, we represent ExprValueMap as a mapping from SCEV
to ValueOffsetPair. We will save both S1->{V1, 0} and S2->{V1, C_a}
into the ExprValueMap when we create SCEV for V1. When S3 is
expanded, it will first expand S2 to V1 - C_a because of S2->{V1,
C_a} in the map, then expand S3 to V1 - C_a + C_b.

Differential Revision: https://reviews.llvm.org/D21313

Fix the runtime error caused by "Use ValueOffsetPair to enhance value
reuse during SCEV expansion".

The patch is to fix the bug in PR28705. It was caused by setting
wrong return value for SCEVExpander::findExistingExpansion. The
return values of findExistingExpansion have different meanings when
the function is used in different ways so it is easy to make mistake.
The fix creates two new interfaces to replace
SCEVExpander::findExistingExpansion, and specifies where each
interface is expected to be used.

Differential Revision: https://reviews.llvm.org/D22942

Create a getelementptr instead of sub expr for ValueOffsetPair if the
value is a pointer.

This patch is to fix PR30213. When expanding an expr based on
ValueOffsetPair, if the value is of pointer type, we can only create
a getelementptr instead of sub expr.

Differential Revision: https://reviews.llvm.org/D24088