Eliminate global pointers that are only stored a single value and null if

Eliminate global pointers that are only stored a single value and null if
we know that all uses of the global will trap if the pointer contained is
null.  In this case, we forward substitute the stored value to any uses.

This has the effect of devirtualizing trivial globals in trivial cases.  For
example, 164.gzip contains this:

gzip.h:extern   int (*read_buf) OF((char *buf, unsigned size));
bits.c: read_buf  = file_read;
deflate.c:    lookahead = read_buf((char*)window,
deflate.c:        n = read_buf((char*)window+strstart+lookahead, more);

Since read_buf has to point to file_read at every use, we just replace
the calls through read_buf with a direct call to file_read.

This occurs in several benchmarks, including 176.gcc and 164.gzip.  Direct
calls are good and stuff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16871 91177308-0d34-0410-b5e6-96231b3b80d8