For large constants (e.g. arrays and structs with many elements) just

creating the keys and doing comparisons to index into 'Map' takes a lot
of time.  For these large constants, keep an inverse map so that 'remove'
and move operations are much faster.

This speeds up a release build of the bc reader on Eric's nasty python
bytecode file from 1:39 to 1:00s.

llvm-svn: 23624