Allow similar TargetOpcodes to use inheritance to factor out commonality. NFC.

Summary:
While implementing atomicrmw in https://reviews.llvm.org/D40092 I found that
inheritance is unusable for all the Generic Opcodes in GlobalISel. This is
because the whole header is included inside a 'let mayLoad = 0, mayStore = 0 ... in'
block. In TableGen, the order of precedence for field assignments is:
  1. Values from classes the record inherits from.
  2. Values from 'let Name=Value in { ... }'
  3. Values from 'let Name=Value;'
As such the 'let mayLoad = 0, mayStore = 0, ... in' surrounding the
'include "GenericOpcodes.td"' was overriding any values provided via inheritance.
We hadn't noticed this before because we were only using 'let Name=Value;' to
specialize opcodes.

Fix this by moving the default values to the lowest precedence. This is
accomplished by moving the values to a common base class
(StandardPseudoInstruction for most TargetOpcodes, and GenericOpcode for
GlobalISel specific TargetOpcodes)

Reviewers: qcolombet

Reviewed By: qcolombet

Subscribers: llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D40096

llvm-svn: 319701