[X86] Don't turn any_extend from a mask register into a sign_extend during...

[X86] Don't turn any_extend from a mask register into a sign_extend during lowering. Add patterns to match any_extend during isel instead.

SimplifyDemandedBits can turn a sign_extend back into an any_extend and trigger an infinite loop. So instead legalize it the same way as a sign_extend, but preserve the opcode. Then just pattern match it the same as sign_extend during isel.

I don't have a reduced test case for such an infinite loop yet.

llvm-svn: 346170