Define more SchedWrites for annotating X86 instructions.

Since almost all X86 instructions can fold loads, use a multiclass to
define register/memory pairs of SchedWrites.

An X86FoldableSchedWrite represents the register version of an
instruction. It holds a reference to the SchedWrite to use when the
instruction folds a load.

This will be used inside multiclasses that define rr and rm instruction
versions together.