Replace archive members in the old position.

This matches gnu archive behavior and since archive member order can change
which member is used, not changing the order on replacement looks like the
right thing to do.

This patch also refactors the logic for which archive member to keep and
whether to move it to a helper function (computeInsertAction). The
nesting in computeNewArchiveMembers was getting a bit confusing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186829 91177308-0d34-0410-b5e6-96231b3b80d8
This adds range checking for "ldr Rn, [pc, #imm]" Thumb 
instructions. With this patch:

1. ldr.n is recognized as mnemonic for the short encoding
2. ldr.w is recognized as menmonic for the long encoding
3. ldr will map to either short or long encodings depending on the size of the offset

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186831 91177308-0d34-0410-b5e6-96231b3b80d8