Improve sinking pass
This commit does two things: * Allows duplicating of global registers and literals by inlining them. Previously we would only inline global register or literal if it was used only once. * Changes method of determining conflicts between a node and an assignment. New method has two advantages. It relies on DefinerOfRegs and UserOfRegs typeclasses, so if a set of registers defined or used by a node should ever change, `conflicts` function will use the changed definition. This definition also catches more cases than the previous one (namely CmmCall and CmmForeignCall) which is a step towards making it possible to run sinking pass before stack layout (currently this doesn't work). This patch also adds a lot of comments that are result of about two-week long investigation of how sinking pass works and why it does what it does.