Speed up Value::isUsedInBasicBlock() for long use lists.

This is expanding Ben's original heuristic for short basic blocks to
also work for longer basic blocks and huge use lists.

Scan the basic block and the use list in parallel, terminating the
search when the shorter list ends. In almost all cases, either the basic
block or the use list is short, and the function returns quickly.

In one crazy test case with very long use chains, CodeGenPrepare runs
400x faster. When compiling ARMDisassembler.cpp it is 5x faster.

<rdar://problem/13840497>