The 32-bit displacement field in an x86 address is signed. Arrange for it

The 32-bit displacement field in an x86 address is signed. Arrange for it
to be sign-extended when it is promoted to 64 bits for intermediate
offset calculations. The offset calculations are done as uint64_t so that
overflow conditions are well defined.

This fixes a problem which is currently hidden by the x86 AsmPrinter but
which was exposed by r58917 (which is temporarily reverted).  See PR3027
for details.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59044 91177308-0d34-0410-b5e6-96231b3b80d8