Win64: Put a REX_W prefix on all TAILJMP* instructions

Win64: Put a REX_W prefix on all TAILJMP* instructions

MSDN's x64 software conventions page says that this is one of the fixed
list of legal epilogues:
https://msdn.microsoft.com/en-us/library/tawsa7cb.aspx

Presumably this is how the unwinder distinguishes epilogue jumps from
in-function control flow.

Also normalize the way we place "## TAILCALL" comments on such jumps.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227611 91177308-0d34-0410-b5e6-96231b3b80d8