[InstCombine] Non-canonical clamp-like pattern handling

Summary:
Given a pattern like:
```
%old_cmp1 = icmp slt i32 %x, C2
%old_replacement = select i1 %old_cmp1, i32 %target_low, i32 %target_high
%old_x_offseted = add i32 %x, C1
%old_cmp0 = icmp ult i32 %old_x_offseted, C0
%r = select i1 %old_cmp0, i32 %x, i32 %old_replacement
```
it can be rewritten as more canonical pattern:
```
%new_cmp1 = icmp slt i32 %x, -C1
%new_cmp2 = icmp sge i32 %x, C0-C1
%new_clamped_low = select i1 %new_cmp1, i32 %target_low, i32 %x
%r = select i1 %new_cmp2, i32 %target_high, i32 %new_clamped_low
```
Iff `-C1 s<= C2 s<= C0-C1`
Also, `ULT` predicate can also be `UGE`; or `UGT` iff `C0 != -1` (+invert result)
Also, `SLT` predicate can also be `SGE`; or `SGT` iff `C2 != INT_MAX` (+invert result)

If `C1 == 0`, then all 3 instructions must be one-use; else at most either `%old_cmp1` or `%old_x_offseted` can have extra uses.
NOTE: if we could reuse `%old_cmp1` as one of the comparisons we'll have to build, this could be less limiting.

So there are two icmp's, each one with 3 predicate variants, so there are 9 fold variants:

|     | ULT                            | UGE                             | UGT                             |
| SLT | https://rise4fun.com/Alive/yIJ | https://rise4fun.com/Alive/5BfN | https://rise4fun.com/Alive/INH  |
| SGE | https://rise4fun.com/Alive/hd8 | https://rise4fun.com/Alive/Abk  | https://rise4fun.com/Alive/PlzS |
| SGT | https://rise4fun.com/Alive/VYG | https://rise4fun.com/Alive/oMY  | https://rise4fun.com/Alive/KrzC |
{F9730206}

This fold was brought up in https://reviews.llvm.org/D65148#1603922 by @dmgreen, and is needed to unblock that patch.
This patch requires D65530.

Reviewers: spatel, nikic, xbolva00, dmgreen

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits, dmgreen

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65765

llvm-svn: 368687

0 files changed, 0 insertions, 0 deletions