Merging r294527:

Merging r294527:
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r294527 | arnolds | 2017-02-08 14:30:47 -0800 (Wed, 08 Feb 2017) | 14 lines

[ARM/AArch ISel] SwiftCC: First parameters that are marked swiftself are not 'this returns'

We mark X0 as preserved by a call that passes the returned parameter.

 x0 = ...
 fun(x0) // no implicit def of x0

This no longer is valid if we pass the parameter in a different register then
the returned value as is the case with a swiftself parameter (passed in x20).

x20 = ...
fun(x20) // there should be an implict def of x8

rdar://30425845
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_40@295135 91177308-0d34-0410-b5e6-96231b3b80d8