auto merge of #18080 : veddan/rust/assume, r=thestinger

Adds an `assume` intrinsic that gets translated to llvm.assume. It is
used on a boolean expression and allows the optimizer to assume that
the expression is true.

This implements #18051.

diff --cc src/libcore/intrinsics.rs
Simple merge

diff --cc src/librustc/middle/trans/context.rs
Simple merge

diff --cc src/librustc/middle/trans/intrinsic.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge