[LoopDist] Add llvm.loop.distribute.enable loop metadata

Description

[LoopDist] Add llvm.loop.distribute.enable loop metadata

Summary:
D19403 adds a new pragma for loop distribution. This change adds
support for the corresponding metadata that the pragma is translated to
by the FE.

As part of this I had to rethink the flag -enable-loop-distribute. My
goal was to be backward compatible with the existing behavior:

A1. pass is off by default from the optimization pipeline
unless -enable-loop-distribute is specified

A2. pass is on when invoked directly from opt (e.g. for unit-testing)

The new pragma/metadata overrides these defaults so the new behavior is:

B1. A1 + enable distribution for individual loop with the pragma/metadata

B2. A2 + disable distribution for individual loop with the pragma/metadata

The default value whether the pass is on or off comes from the initiator
of the pass. From the PassManagerBuilder the default is off, from opt
it's on.

I moved -enable-loop-distribute under the pass. If the flag is
specified it overrides the default from above.

Then the pragma/metadata can further modifies this per loop.

As a side-effect, we can now also use -enable-loop-distribute=0 from opt
to emulate the default from the optimization pipeline. So to be precise
this is the new behavior:

C1. pass is off by default from the optimization pipeline
unless -enable-loop-distribute or the pragma/metadata enables it

C2. pass is on when invoked directly from opt
unless -enable-loop-distribute=0 or the pragma/metadata disables it

Reviewers: hfinkel

Subscribers: joker.eph, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D19431

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