[RISCV] CSE by swapping conditional branches (#71111)

DAGCombiner, as well as InstCombine, tend to canonicalize GE/LE into
GT/LT, namely:
```
X >= C --> X > (C - 1)
```
Which sometime generates off-by-one constants that could have been CSE'd
with surrounding constants.
Instead of changing such canonicalization, this patch tries to swap
those branch conditions post-isel, in the hope of resurfacing more
constant CSE opportunities. More specifically, it performs the following
optimization:

For two constants C0 and C1 from
```
li Y, C0
li Z, C1
```
To remove redundnat `li Y, C0`,
 1. if C1 = C0 + 1 we can turn: 
    (a) blt Y, X -> bge X, Z
    (b) bge Y, X -> blt X, Z
 2. if C1 = C0 - 1 we can turn: 
    (a) blt X, Y -> bge Z, X
    (b) bge X, Y -> blt Z, X

This optimization will be done by PeepholeOptimizer through
RISCVInstrInfo::optimizeCondBranch.

0 files changed, 0 insertions, 0 deletions