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[Inliner] Add a flag to disable manual alloca merging in the Inliner.

Description

[Inliner] Add a flag to disable manual alloca merging in the Inliner.

This is off for now while testing can take place to make sure that in
fact we do sufficient stack coloring to fully obviate the manual alloca
array merging.

Some context on why we should be using stack coloring rather than
merging allocas in this way:

LLVM relies very heavily on analyzing pointers as coming from different
allocas in order to make aliasing decisions. These are some of the most
powerful aliasing signals available in LLVM. So merging allocas is an
extremely destructive operation on the LLVM IR -- it takes away highly
valuable and hard to reconstruct information.

As a consequence, inlined functions which happen to have array allocas
that this pattern matches will fail to be properly interleaved unless
SROA manages to hoist everything to an SSA register. Instead, the
inliner will have added an unnecessary dependence that one inlined
function execute after the other because they will have been rewritten
to refer to the same memory.

All that said, folks will reasonably want some time to experiment here
and make sure there are no significant regressions. A flag should give
us an easy knob to test.

For more context, see the thread here:
http://lists.llvm.org/pipermail/llvm-dev/2016-July/103277.html
http://lists.llvm.org/pipermail/llvm-dev/2016-August/103285.html

Differential Revision: https://reviews.llvm.org/D23052

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