Auto merge of rust-lang#35261 - eddyb:llvm-autoupgrade, r=nikomatsakis

Support removed LLVM intrinsics by invoking its AutoUpgrade mechanism.

Turns out that LLVM sometimes renames platform intrinsics or replaces them with first-class instructions.
For example, signed minimum became `select (icmp SLT, a, b), a, b` where `a` and `b` are vectors.

This is blocking the Servo rustup ([relevant failure](http://build.servo.org/builders/windows-dev/builds/226/steps/compile/logs/stdio)), as they're using a few such intrinsics.
The fix in this PR is to invoke LLVM's own `AutoUpgrade` mechanism to do the replacements.

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