Subzero x86-64 call instruction operands can be 64-bit now and require
a register.

Bug chromium:860533

Change-Id: I4446be34f57ef73e24718252210bdaa81995c8be
Reviewed-on: https://swiftshader-review.googlesource.com/19848Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>