[RegAllocGreedy] Provide a flag to disable the local reassignment heuristic.
By default, no functionality change.

Before evicting a local variable, this heuristic tries to find another (set of)
local(s) that can be reassigned to a free color.

In some extreme cases (large basic blocks with tons of local variables), the
compilation time is dominated by the local interference checks that this
heuristic must perform, with no code gen gain.
E.g., the motivating example takes 4 minutes to compile with this heuristic, 12
seconds without.

Improving the situation will likely require to make drastic changes to the
register allocator and/or the interference check framework.

For now, provide this flag to better understand the impact of that heuristic.

<rdar://problem/17444599>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212099 91177308-0d34-0410-b5e6-96231b3b80d8
[PeepholeOptimizer] Advanced rewriting of copies to avoid cross register banks
copies.

This patch extends the peephole optimization introduced in r190713 to produce
register-coalescer friendly copies when possible.

This extension taught the existing cross-bank copy optimization how to deal
with the instructions that generate cross-bank copies, i.e., insert_subreg,
extract_subreg, reg_sequence, and subreg_to_reg.
E.g.
b = insert_subreg e, A, sub0 <-- cross-bank copy
...
C = copy b.sub0 <-- cross-bank copy

Would produce the following code:
b = insert_subreg e, A, sub0 <-- cross-bank copy
...
C = copy A <-- same-bank copy

This patch also introduces a new helper class for that: ValueTracker.
This class implements the logic to look through the copy related instructions
and get the related source.

For now, the advanced rewriting is disabled by default as we are lacking the
semantic on target specific instructions to catch the motivating examples.

Related to <rdar://problem/12702965>.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212100 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid revocations when possible.

This is a small targeted fix for pr20119. The code needs quiet a bit of
refactoring and I added some FIXMEs about it, but I want to get the testcase
passing first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212101 91177308-0d34-0410-b5e6-96231b3b80d8
Rephrase loop so it doesn't leave unused bools around in Release mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212102 91177308-0d34-0410-b5e6-96231b3b80d8