[SCEV] createAddRecFromPHI: Optimize for the most common case.

Summary:
The existing implementation creates a symbolic SCEV expression every
time we analyze a phi node and then has to remove it, when the analysis
is finished. This is very expensive, and in most of the cases it's also
unnecessary. According to the data I collected, ~60-70% of analyzed phi
nodes (measured on SPEC) have the following form:
  PN = phi(Start, OP(Self, Constant))
Handling such cases separately significantly speeds this up.

Reviewers: sanjoy, pete

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32663

llvm-svn: 302096