tint: Implement sem::Load

The resolver now wraps sem::Expression objects with a sem::Load object
anywhere that the load rule is invoked. sem::Expression provides an
`UnwrapLoad()` method that returns the inner expression (or
passthrough, if no load is present), which is analaguous to
Type::UnwrapRef().

The logic for alias analysis in `RegisterLoadIfNeeded` has been folded
into the new `Resolver::Load` method.

Fixed up many transforms and tests. The only difference in output is
for a single SPIR-V backend test, where some IDs have changed due to
slight re-ordering of when expressions are generated.

There may be further clean-ups possible (e.g. removing unnecessary
calls to `UnwrapRef`, and simplifying places in the SPIR-V writer or
transforms that deal with memory accesses), but these can be addressed
in future patches.

Fixed: tint:1654
Change-Id: I69adecfe9251faae46546b64d0cdc29eea26cd4e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/99706
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/tint/access/var/vector.wgsl.expected.spvasm

@@ -24,8 +24,8 @@
           %v = OpVariable %_ptr_Function_v3float Function %9
          %13 = OpAccessChain %_ptr_Function_float %v %uint_1
          %14 = OpLoad %float %13
-         %16 = OpLoad %v3float %v
-         %17 = OpVectorShuffle %v2float %16 %16 0 2
+         %15 = OpLoad %v3float %v
+         %17 = OpVectorShuffle %v2float %15 %15 0 2
          %18 = OpLoad %v3float %v
          %19 = OpVectorShuffle %v3float %18 %18 0 2 1
                OpReturn