Canonicalization for @llvm.assume

Adds simple logical canonicalization of assumption intrinsics to instcombine,
currently:
 - invariant(a && b) -> invariant(a); invariant(b)
 - invariant(!(a || b)) -> invariant(!a); invariant(!b)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213977 91177308-0d34-0410-b5e6-96231b3b80d8