[InstCombine] Remove trivially empty lifetime start/end ranges.

Summary:
Some passes may open up opportunities for optimizations, leaving empty
lifetime start/end ranges. For example, with the following code:

    void foo(char *, char *);
    void bar(int Size, bool flag) {
      for (int i = 0; i < Size; ++i) {
        char text[1];
        char buff[1];
        if (flag)
          foo(text, buff); // BBFoo
      }
    }

the loop unswitch pass will create 2 versions of the loop, one with
flag==true, and the other one with flag==false, but always leaving
the BBFoo basic block, with lifetime ranges covering the scope of the for
loop. Simplify CFG will then remove BBFoo in the case where flag==false,
but will leave the lifetime markers.

This patch teaches InstCombine to remove trivially empty lifetime marker
ranges, that is ranges ending right after they were started (ignoring
debug info or other lifetime markers in the range).

This fixes PR24598: excessive compile time after r234581.

Reviewers: reames, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13305

llvm-svn: 249018