Move the ValueMapT declaration out of BlockGenerator

  Value maps are created and used in many places and it is not always
  possible to include CodeGen/Blockgenerators.h. To this end, ValueMapT
  now lives in the ScopHelper.h which does not have any dependences itself.

  This patch also replaces uses of different other value map types with
  the ValueMapT.

llvm-svn: 249606