[InstSimplify] shl nuw C, %x -> C iff signbit is set on C.

Summary:
`%r = shl nuw i8 C, %x`

As per langref:
```
If the nuw keyword is present, then the shift produces
a poison value if it shifts out any non-zero bits.
```
Thus, if the sign bit is set on `C`, then `%x` can only be `0`,
which means that `%r` can only be `C`.
Or in other words, set sign bit means that the signed value
is negative, so the constant is `<= 0`.

https://rise4fun.com/Alive/WMk
https://rise4fun.com/Alive/udv

Was mentioned in D47428 review.

We already handle the `0` constant, https://godbolt.org/g/UZq1sJ, so this only handles negative constants.

Could use computeKnownBits() / LazyValueInfo,
but the cost-benefit analysis (https://reviews.llvm.org/D47891)
suggests it isn't worth it.

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47883

llvm-svn: 334222