llvm.experimental.stackmap: fix encoding of large constants.

In the stackmap format we advertise the constant field as signed.
However, we were determining whether to promote to a 64-bit constant
pool based on an unsigned comparison.

This fix allows -1 to be encoded as a small constant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198816 91177308-0d34-0410-b5e6-96231b3b80d8