Add support for 64-bit calling convention.

This is far from complete, but it is enough to make it possible to write
test cases using i64 arguments.

Missing features:
- Floating point arguments.
- Receiving arguments on the stack.
- Calls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178523 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/SPARC/64bit.ll

@@ -0,0 +1,7 @@
+; RUN: llc < %s -march=sparcv9 | FileCheck %s
+
+; CHECK: ret2:
+; CHECK: or %g0, %i1, %i0
+define i64 @ret2(i64 %a, i64 %b) {
+  ret i64 %b
+}