[X86] Emit BZHI when mask is ~(-1 << nbits))

Summary:
In D47428, i propose to choose the `~(-(1 << nbits))` as the canonical form of low-bit-mask formation.
As it is seen from these tests, there is a reason for that.

AArch64 currently better handles `~(-(1 << nbits))`, but not the more traditional `(1 << nbits) - 1` (sic!).
The other way around for X86.
It would be much better to canonicalize.

This patch is completely monkey-typing.
I don't really understand how this works :)
I have based it on `// x & (-1 >> (32 - y))` pattern.

Also, when we only have `BMI`, i wonder if we could use `BEXTR` with `start=0` ?

Related links:
https://bugs.llvm.org/show_bug.cgi?id=36419
https://bugs.llvm.org/show_bug.cgi?id=37603
https://bugs.llvm.org/show_bug.cgi?id=37610
https://rise4fun.com/Alive/idM

Reviewers: craig.topper, spatel, RKSimon, javed.absar

Reviewed By: craig.topper

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D47453

llvm-svn: 334125