Statepoint: Fix handling of Far Immediate calls

gc.statepoint intrinsics with a far immediate call target 
were lowered incorrectly as pc-rel32 calls.

This change fixes the problem, and generates an indirect call 
via a scratch register.

For example: 

Intrinsic:
  %safepoint_token = call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* inttoptr (i64 140727162896504 to void ()*), i32 0, i32 0, i32 0, i32 0)

Old Incorrect Lowering:
  callq 140727162896504

New Correct Lowering:
  movabsq $140727162896504, %rax 
  callq *%rax

In lowerCallFromStatepoint(), the callee-target was modified and 
represented as a "TargetConstant" node, rather than a "Constant" node.
Undoing this modification enabled LowerCall() to generate the 
correct CALL instruction.

llvm-svn: 239114