Don't unconditionally suppress hoisting of instructions with implicit

Don't unconditionally suppress hoisting of instructions with implicit
defs or uses. The regular def and use checking below covers them, and
can be more precise. It's safe to hoist an instruction with a dead
implicit def if the register isn't live into the loop header.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@97352 91177308-0d34-0410-b5e6-96231b3b80d8