Mark &mut pointers as noalias once LLVM no longer miscompiles them

[bstrie]

This issue tracks the undoing of the workaround introduced in #31545 on account of a bug in LLVM. cc @dotdash

[jplatte]

Was just reading about this, and it was a little hard to find the LLVM bug report, so for reference, here it is.

[eefriedman]

I've been working through the LLVM optimization passes to find noalias-related bugs. Known issues:

• https://bugs.llvm.org/show_bug.cgi?id=25422 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27849 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27857 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27858 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27859 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27860 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27952 Fixed
• https://bugs.llvm.org/show_bug.cgi?id=27953 Fixed

Some of these are less important because they can't caused a wrong value, only a segfault.

There are probably a few more lurking issues, but hopefully I've found most of them.

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(Edited by bluss 2016-12-15 to strike out fixed bug)
(Edited by mbrubeck 2017-03-30 to link to new LLVM bugzilla domain)
(Edited by scottmcm 2018-03-29 to strike out fixed bug)

[mrhota]

@eefriedman LLVM bug 27859 is closed.