Change the signature of insertElementAt and rename addInputElementFront

insertElementAt(x, END) does the identical thing as addInputElement(x),
so the only reasonable use of insertElementAt is to call it with the
other possible argument, BEGIN. That means the second parameter of the
function is just redundant. This patch is to remove the second
parameter and rename the function accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213821 91177308-0d34-0410-b5e6-96231b3b80d8