This is in preparation for refactoring all x64::Inst arms to use OperandSize.
Current uses of OperandSize fall into two categories:
1. XMM operations which require 32/64 bit operands
2. Immediates which only care about 64-bit or not.
Adds assertions to existing Inst constructors to check that they are passed valid sizes.
This change also removes the implicit widening of 1 and 2 byte values to 4 bytes. from_bytes() is only used by category 2, so removing this behavior will not change any visible behavior.
Overall this change should be a no-op.