[ARM] Remove trunc sinks in ARM CGP

    
Truncs are treated as sources if their produce a value of the same
type as the one we currently trying to promote. Truncs used to be
considered as a sink if their operand was the same value type.
    
We now allow smaller types in the search, so we should search through
truncs that produce a smaller value. These truncs can then be
converted to an AND mask.
    
This leaves sinks as being:
  - points where the value in the register is being observed, such as
    an icmp, switch or store.
  - points where value types have to match, such as calls and returns.
  - zext are included to ease the transformation and are generally
    removed later on.
    
During this change, it also became apart from truncating sinks was
broken: if a sink used a source, its type information had already
been lost by the time the truncation happens. So I've changed the
method of caching the type information.

Differential Revision: https://reviews.llvm.org/D54515

llvm-svn: 347191