[InstSimplify] Drop leftover "division-by-zero guard" around...

[InstSimplify] Drop leftover "division-by-zero guard" around `@llvm.umul.with.overflow` overflow bit

Summary:
Now that with D65143/D65144 we've produce `@llvm.umul.with.overflow`,
and with D65147 we've flattened the CFG, we now can see that
the guard may have been there to prevent division by zero is redundant.
We can simply drop it:
```
----------------------------------------
Name: no overflow and not zero
  %iszero = icmp ne i4 %y, 0
  %umul = umul_overflow i4 %x, %y
  %umul.ov = extractvalue {i4, i1} %umul, 1
  %retval.0 = and i1 %iszero, %umul.ov
  ret i1 %retval.0
=>
  %iszero = icmp ne i4 %y, 0
  %umul = umul_overflow i4 %x, %y
  %umul.ov = extractvalue {i4, i1} %umul, 1
  %retval.0 = and i1 %iszero, %umul.ov
  ret %umul.ov

Done: 1
Optimization is correct!
```

Reviewers: nikic, spatel, xbolva00

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65150

llvm-svn: 370350