If a variable can only hold two values, and is not already a bool, shrink it

If a variable can only hold two values, and is not already a bool, shrink it
down to actually BE a bool.  This allows simple value range propagation
stuff work harder, deleting comparisons in bzip2 in some hot loops.

This implements GlobalOpt/integer-bool.ll, which is the essence of the
loop condition distilled into a testcase.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18817 91177308-0d34-0410-b5e6-96231b3b80d8