[InstCombine] Optimize icmp slt signum(x), 1 --> icmp slt x, 1

Summary:
`signum(x)` is sometimes implemented as `(x >> 63) | (-x >>> 63)` (for
an `i64` `x`).  This change adds a matcher for that pattern, and an
instcombine rule to optimize `signum(x) s< 1`.

Later, we can also consider optimizing:

  icmp slt signum(x), 0 --> icmp slt x, 0
  icmp sle signum(x), 1 --> true

etc.

Reviewers: majnemer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12703

llvm-svn: 247846