this logic is broken for trunc to bool, replace the folding logic for trunc

this logic is broken for trunc to bool, replace the folding logic for trunc
completely, as it is trivial.  We should probably do this for the rest of the
cast operations.  This fixes ConstProp/2006-12-01-TruncBoolBug.ll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32081 91177308-0d34-0410-b5e6-96231b3b80d8