Change ConstantArray::replaceUsesOfWithOnConstant to attempt to update

constant arrays in place instead of reallocating them and replaceAllUsesOf'ing
the result.  This speeds up a release build of the bcreader from:

136.987u 120.866s 4:24.38
to
49.790u 49.890s 1:40.14

... a 2.6x speedup parsing a large python bc file.

llvm-svn: 23614