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/aarch64/inst/unwind.rs

@@ -29,6 +29,7 @@ impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
                     rt,
                     rt2,
                     mem: PairAMode::PreIndexed(rn, imm7),
+                    ..
                 } if *rt == regs::fp_reg()
                     && *rt2 == regs::link_reg()
                     && *rn == regs::writable_stack_reg()
@@ -60,6 +61,7 @@ impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
                     rt,
                     rt2,
                     mem: PairAMode::PreIndexed(rn, imm7),
+                    ..
                 } if rn.to_reg() == regs::stack_reg() && imm7.value % (pair_size as i16) == 0 => {
                     // stp r1, r2, [sp, #(i * #16)]
                     let stack_offset = imm7.value as u32;