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/abi.rs

@@ -1,7 +1,7 @@
 //! Implementation of the standard x64 ABI.
 
 use crate::ir::types::*;
-use crate::ir::{self, types, TrapCode, Type};
+use crate::ir::{self, types, MemFlags, TrapCode, Type};
 use crate::isa;
 use crate::isa::{x64::inst::*, CallConv};
 use crate::machinst::abi_impl::*;
@@ -618,6 +618,7 @@ impl From<StackAMode> for SyntheticAmode {
                 SyntheticAmode::Real(Amode::ImmReg {
                     simm32,
                     base: regs::rbp(),
+                    flags: MemFlags::trusted(),
                 })
             }
             StackAMode::NominalSPOffset(off, _ty) => {
@@ -634,6 +635,7 @@ impl From<StackAMode> for SyntheticAmode {
                 SyntheticAmode::Real(Amode::ImmReg {
                     simm32,
                     base: regs::rsp(),
+                    flags: MemFlags::trusted(),
                 })
             }
         }