X86: Use a callee save register for the swiftself parameter.
It is very likely that the swiftself parameter is alive throughout most functions function so putting it into a callee save register should avoid spills for the callers with only a minimum amount of extra spills in the callees. Currently the generated code is correct but unnecessarily spills and reloads arguments passed in callee save registers, I will address this in upcoming patches. This also adds a missing check that for tail calls the preserved value of the caller must be the same as the callees parameter. Differential Revision: http://reviews.llvm.org/D18902 llvm-svn: 266252
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