[Loop Fusion] Integrate Loop Peeling into Loop Fusion

Summary:
This patch adds the ability to peel off iterations of the first loop in loop
fusion. This can allow for both loops to have the same trip count, making it
legal for them to be fused together.

Here is a simple scenario peeling can be used in loop fusion:

for (i = 0; i < 10; ++i)
  a[i] = a[i] + 3;
for (j = 1; j < 10; ++j)
  b[j] = b[j] + 5;

Here is we can make use of peeling, and then fuse the two loops together. We can
peel off the 0th iteration of the loop i, and then combine loop i and j for
i = 1 to 10.

a[0] = a[0] +3;
for (i = 1; i < 10; ++i) {
  a[i] = a[i] + 3;
  b[i] = b[i] + 5;
}

Currently peeling with loop fusion is only supported for loops with constant
trip counts and a single exit point. Both unguarded and guarded loops are
supported.

Author: sidbav (Sidharth Baveja)

Reviewers: kbarton, Meinersbur, bkramer, Whitney, skatkov, ashlykov, fhahn, bmahjour

Reviewed By: bmahjour

Subscribers: bmahjour, mgorny, hiraditya, zzheng

Tags: LLVM

Differential Revision: https://reviews.llvm.org/D82927

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