[TableGen] Examine entire subreg compositions to detect ambiguity

When tablegen detects that there exist two subregister compositions that
result in the same value for some register, it will emit a warning. This
kind of an overlap in compositions should only happen when it is caused
by a user-defined composition. It can happen, however, that the user-
defined composition is not identically equal to another one, but it does
produce the same value for one or more registers. In such cases suppress
the warning.
This patch is to silence the warning when building the System Z backend
after D50725.

Differential Revision: https://reviews.llvm.org/D50977

llvm-svn: 347894