Dont use rAsm implicitly in x64Assembler
This removes the implicit uses of rAsm from X64Assembler,
and moves the definition to abi-x64.h. Some mentions of
rAsm were hoisted, but most involved doing stores, so those
were converted to store two 32-bit immediates (without
using a scratch register) when possible.
The Immed wrapper class now only allows 32-bit immediates,
and a new Immed64 wrapper is available for immediates whose
compile-time-type (vm-compile time) is 64 bits.
Reviewed By: @ottoni
Differential Revision:
D1224706