LoopIdiomRecognize: add negative tests for powi idiom

The following code, when compiled under -ffast-math, produces bad
codegen due to LoopVectorize:

  float powi(float base, int exp) {
    float result = 1.0;
      for (int i = 0; i < exp; ++i)
        result *= base;
    return result;
  }

It can easily be replaced with the llvm.powi intrinsic, when the
exponent is a C int type. This is the job of LoopIdiomRecognize, and has
been marked as a TODO item for years. In preparation to fulfill this
wish, add negative tests corresponding to variations of this program.

Author: artagnon