Change ConstantArray::replaceUsesOfWithOnConstant to attempt to update

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23614 91177308-0d34-0410-b5e6-96231b3b80d8