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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llvm-svn: 295135