x64 and aarch64: carry MemFlags on loads/stores; don't emit trap info unless an op can trap.

This end result was previously enacted by carrying a `SourceLoc` on
every load/store, which was somewhat cumbersome, and only indirectly
encoded metadata about a memory reference (can it trap) by its presence
or absence. We have a type for this -- `MemFlags` -- that tells us
everything we might want to know about a load or store, and we should
plumb it through to code emission instead.

This PR attaches a `MemFlags` to an `Amode` on x64, and puts it on load
and store `Inst` variants on aarch64. These two choices seem to factor
things out in the nicest way: there are relatively few load/store insts
on aarch64 but many addressing modes, while the opposite is true on x64.

cranelift/codegen/src/isa/x64/inst/unwind.rs

@@ -61,7 +61,7 @@ impl UnwindInfoGenerator<Inst> for X64UnwindInfo {
                 }
                 Inst::MovRM {
                     src,
-                    dst: SyntheticAmode::Real(Amode::ImmReg { simm32, base }),
+                    dst: SyntheticAmode::Real(Amode::ImmReg { simm32, base, .. }),
                     ..
                 } if *base == regs::rsp() => {
                     // `mov reg, imm(rsp)`