[InstCombine] Fold `and(shl(zext(x), width(SIGNMASK) - width(%x)), SIGNMASK)` to...

[InstCombine] Fold `and(shl(zext(x), width(SIGNMASK) - width(%x)), SIGNMASK)` to `and(sext(%x), SIGNMASK)`

One less instruction and reducing use count of zext.
As alive2 confirms, we're fine with all the weird combinations of
undef elts in constants, but unless the shift amount was undef
for a lane, we must sanitize undef mask to zero, since sign bits
are no longer zeros.

https://rise4fun.com/Alive/d7r
```
----------------------------------------
Optimization: zz
Precondition: ((C1 == (width(%r) - width(%x))) && isSignBit(C2))
  %o0 = zext %x
  %o1 = shl %o0, C1
  %r = and %o1, C2
=>
  %n0 = sext %x
  %r = and %n0, C2

Done: 2016
Optimization is correct!
```