[GlobalISel] Add matchers for specific constants and a matcher for negations

It's fairly common to need matchers for a specific constant value, or for
common idioms like finding a negated register.

Add

- `m_SpecificICst`, which returns true when matching a specific value..
- `m_ZeroInt`, which returns true when an integer 0 is matched.
- `m_Neg`, which returns when a register is negated.

Also update a few places which use idioms related to the new matchers.

Differential Revision: https://reviews.llvm.org/D91397