[X86] Add option to specify preferable loop alignment

Summary:
Loop alignment can cause a significant change of
the perfromance for short loops.
To be able to evaluate the impact of loop alignment this change
introduces the new option x86-experimental-pref-loop-alignment.
The alignment will be 2^Value bytes, the default value is 4.

Patch by Serguei Katkov!

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D30391

llvm-svn: 297178