Auto merge of #35300 - eddyb:mir-neg-overflow, r=arielb1

rustc_trans: don't Assert(Overflow(Neg)) when overflow checks are off.

Generic functions using `Neg` on primitive types would panic even in release mode, with MIR trans.
The solution is a bit hacky, as I'm checking the message, since there's no dedicated `CheckedUnOp`.

Blocks Servo rustup ([failure #1](http://build.servo.org/builders/linux-rel/builds/2477/steps/test_3/logs/stdio), [failure #2](http://build.servo.org/builders/mac-rel-css/builds/2364/steps/test/logs/stdio)) - this should be the last hurdle, it affects only one test.