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LoopVectorize: MaxVF should not be larger than the loop trip count

Description

LoopVectorize: MaxVF should not be larger than the loop trip count

Summary:
Improve how MaxVF is computed while taking into account that MaxVF should not be larger than the loop's trip count.

Other than saving on compile-time by pruning the possible MaxVF candidates, this patch fixes pr34438 which exposed the following flow:

  1. Short trip count identified -> Don't bail out, set OptForSize:=True to avoid tail-loop and runtime checks.
  2. Compute MaxVF returned 16 on a target supporting AVX512.
  3. OptForSize -> choose VF:=MaxVF.
  4. Bail out because TripCount = 8, VF = 16, TripCount % VF !=0 means we need a tail loop.

With this patch step 2. will choose MaxVF=8 based on TripCount.

Reviewers: Ayal, dorit, mkuper, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D37425

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