[InstCombine] Fold '(-1 u/ %x) u< %y' to '@llvm.umul.with.overflow' + overflow bit extraction

Summary:
`(-1 u/ %x) u< %y` is one of (3?) common ways to check that
some unsigned multiplication (will not) overflow.
Currently, we don't catch it. We could:
```
----------------------------------------
Name: no overflow
  %o0 = udiv i4 -1, %x
  %r = icmp ult i4 %o0, %y
=>
  %o0 = udiv i4 -1, %x
  %n0 = umul_overflow i4 %x, %y
  %r = extractvalue {i4, i1} %n0, 1

Done: 1
Optimization is correct!

----------------------------------------
Name: no overflow, swapped
  %o0 = udiv i4 -1, %x
  %r = icmp ugt i4 %y, %o0
=>
  %o0 = udiv i4 -1, %x
  %n0 = umul_overflow i4 %x, %y
  %r = extractvalue {i4, i1} %n0, 1

Done: 1
Optimization is correct!

----------------------------------------
Name: overflow
  %o0 = udiv i4 -1, %x
  %r = icmp uge i4 %o0, %y
=>
  %o0 = udiv i4 -1, %x
  %n0 = umul_overflow i4 %x, %y
  %n1 = extractvalue {i4, i1} %n0, 1
  %r = xor %n1, -1

Done: 1
Optimization is correct!

----------------------------------------
Name: overflow
  %o0 = udiv i4 -1, %x
  %r = icmp ule i4 %y, %o0
=>
  %o0 = udiv i4 -1, %x
  %n0 = umul_overflow i4 %x, %y
  %n1 = extractvalue {i4, i1} %n0, 1
  %r = xor %n1, -1

Done: 1
Optimization is correct!
```

As it can be observed from tests, while simply forming the `@llvm.umul.with.overflow`
is easy, if we were looking for the inverted answer, then more work needs to be done
to cleanup the now-pointless control-flow that was guarding against division-by-zero.
This is being addressed in follow-up patches.

Reviewers: nikic, spatel, efriedma, xbolva00, RKSimon

Reviewed By: nikic, xbolva00

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65143

llvm-svn: 370347