[InstCombine] Try to reuse constant from select in leading comparison

Summary:
If we have e.g.:
```
  %t = icmp ult i32 %x, 65536
  %r = select i1 %t, i32 %y, i32 65535
```
the constants `65535` and `65536` are suspiciously close.
We could perform a transformation to deduplicate them:
```
Name: ult
%t = icmp ult i32 %x, 65536
%r = select i1 %t, i32 %y, i32 65535
  =>
%t.inv = icmp ugt i32 %x, 65535
%r = select i1 %t.inv, i32 65535, i32 %y
```
https://rise4fun.com/Alive/avb

While this may seem esoteric, this should certainly be good for vectors
(less constant pool usage) and for opt-for-size - need to have only one constant.

But the real fun part here is that it allows further transformation,
in particular it finishes cleaning up the `clamp` folding,
see e.g. `canonicalize-clamp-with-select-of-constant-threshold-pattern.ll`.
We start with e.g.
```
  %dont_need_to_clamp_positive = icmp sle i32 %X, 32767
  %dont_need_to_clamp_negative = icmp sge i32 %X, -32768
  %clamp_limit = select i1 %dont_need_to_clamp_positive, i32 -32768, i32 32767
  %dont_need_to_clamp = and i1 %dont_need_to_clamp_positive, %dont_need_to_clamp_negative
  %R = select i1 %dont_need_to_clamp, i32 %X, i32 %clamp_limit
```
without this patch we currently produce
```
  %1 = icmp slt i32 %X, 32768
  %2 = icmp sgt i32 %X, -32768
  %3 = select i1 %2, i32 %X, i32 -32768
  %R = select i1 %1, i32 %3, i32 32767
```
which isn't really a `clamp` - both comparisons are performed on the original value,
this patch changes it into
```
  %1.inv = icmp sgt i32 %X, 32767
  %2 = icmp sgt i32 %X, -32768
  %3 = select i1 %2, i32 %X, i32 -32768
  %R = select i1 %1.inv, i32 32767, i32 %3
```
and then the magic happens! Some further transform finishes polishing it and we finally get:
```
  %t1 = icmp sgt i32 %X, -32768
  %t2 = select i1 %t1, i32 %X, i32 -32768
  %t3 = icmp slt i32 %t2, 32767
  %R = select i1 %t3, i32 %t2, i32 32767
```
which is beautiful and just what we want.

Proofs for `getFlippedStrictnessPredicateAndConstant()` for de-canonicalization:
https://rise4fun.com/Alive/THl
Proofs for the fold itself: https://rise4fun.com/Alive/THl

Reviewers: spatel, dmgreen, nikic, xbolva00

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66232

llvm-svn: 369840