Start trying to make InstCombine preserve more debug info. The idea here is to set...

Start trying to make InstCombine preserve more debug info.  The idea here is to set the debug location on the IRBuilder, which will be then right location in most cases.  This should magically give many transformations debug locations, and fixing places which are missing a debug location will usually just means changing the code creating it to use the IRBuilder.

As an example, the change to InstCombineCalls catches a common case where a call to a bitcast of a function is rewritten.

Chris, does this approach look reasonable?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131516 91177308-0d34-0410-b5e6-96231b3b80d8