[InstCombine] Re-land: Optimize redundant 'signed truncation check pattern'.

Summary:
This comes with `Implicit Conversion Sanitizer - integer sign change` (D50250):
```
signed char test(unsigned int x) { return x; }
```
`clang++ -fsanitize=implicit-conversion -S -emit-llvm -o - /tmp/test.cpp -O3`
* Old: {F6904292}
* With this patch: {F6904294}

General pattern:
  X & Y

Where `Y` is checking that all the high bits (covered by a mask `4294967168`)
are uniform, i.e.  `%arg & 4294967168`  can be either  `4294967168`  or  `0`
Pattern can be one of:
  %t = add        i32 %arg,    128
  %r = icmp   ult i32 %t,      256
Or
  %t0 = shl       i32 %arg,    24
  %t1 = ashr      i32 %t0,     24
  %r  = icmp  eq  i32 %t1,     %arg
Or
  %t0 = trunc     i32 %arg  to i8
  %t1 = sext      i8  %t0   to i32
  %r  = icmp  eq  i32 %t1,     %arg
This pattern is a signed truncation check.

And `X` is checking that some bit in that same mask is zero.
I.e. can be one of:
  %r = icmp sgt i32   %arg,    -1
Or
  %t = and      i32   %arg,    2147483648
  %r = icmp eq  i32   %t,      0

Since we are checking that all the bits in that mask are the same,
and a particular bit is zero, what we are really checking is that all the
masked bits are zero.
So this should be transformed to:
  %r = icmp ult i32 %arg, 128

The transform itself ended up being rather horrible, even though i omitted some cases.
Surely there is some infrastructure that can help clean this up that i missed?

https://rise4fun.com/Alive/3Ou

The initial commit (rL339610)
was reverted, since the first assert was being triggered.
The @positive_with_extra_and test now has coverage for that case.

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: RKSimon, erichkeane, vsk, llvm-commits

Differential Revision: https://reviews.llvm.org/D50465

llvm-svn: 339621