Add support for brff/brif and icmp_sp to new x64 backend to support Lucet.

`lucetc` currently *almost*, but not quite, works with the new x64
backend; the only missing piece is support for the particular
instructions emitted as part of its prologue stack-check.

We do not normally see `brff`, `brif`, or `ifcmp_sp` in CLIF generated by
`cranelift-wasm` without the old-backend legalization rules, so these
were not supported in the new x64 backend as they were not necessary for
Wasm MVP support. Using them resulted in an `unimplemented!()` panic.

This PR adds support for `brff` and `brif` analogously to how AArch64
implements them, by pattern-matching the `ifcmp` / `ffcmp` directly.
Then `ifcmp_sp` is a straightforward variant of `ifcmp`.

Along the way, this also removes the notion of "fallthrough block" from
the branch-group lowering method; instead, `fallthrough` instructions
are handled as normal branches to their explicitly-provided targets,
which (in the original CLIF) match the fallthrough block. The reason for
this is that the block reordering done as part of lowering can change
the fallthrough block. We were not using `fallthrough` instructions in
the output produced by `cranelift-wasm`, so this, too, was not
previously caught.

With these changes, the `lucetc` crate in Lucet passes all tests with
the `x64` feature-flag added to its `cranelift-codegen` dependency.

cranelift/filetests/filetests/isa/x64/branches.clif

@@ -0,0 +1,88 @@
+test compile
+target x86_64
+feature "experimental_x64"
+
+function %f0(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+  v2 = icmp eq v0, v1
+  brnz v2, block1
+  jump block2
+  ; check:  cmpl    %esi, %edi
+  ; nextln: jz      label1; j label2
+
+block1:
+  v3 = iconst.i32 1
+  ; check: movl    $$1, %eax
+  return v3
+  ; check: ret
+
+block2:
+  v4 = iconst.i32 2
+  ; check: movl    $$2, %eax
+  return v4
+  ; check: ret
+}
+
+function %f1(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+  v2 = icmp eq v0, v1
+  brz v2, block1
+  jump block2
+  ; check:  cmpl    %esi, %edi
+  ; nextln: jnz     label1; j label2
+
+block1:
+  v3 = iconst.i32 1
+  ; check: movl    $$1, %eax
+  return v3
+  ; check: ret
+
+block2:
+  v4 = iconst.i32 2
+  ; check: movl    $$2, %eax
+  return v4
+  ; check: ret
+}
+
+function %f2(i32, i32) -> i32 {
+block0(v0: i32, v1: i32):
+  v2 = ifcmp v0, v1
+  brif eq v2, block1
+  jump block2
+  ; check:  cmpl    %esi, %edi
+  ; nextln: jz      label1; j label2
+
+block1:
+  v3 = iconst.i32 1
+  ; check: movl    $$1, %eax
+  return v3
+  ; check: ret
+
+block2:
+  v4 = iconst.i32 2
+  ; check: movl    $$2, %eax
+  return v4
+  ; check: ret
+}
+
+function %f3(f32, f32) -> i32 {
+block0(v0: f32, v1: f32):
+  v2 = ffcmp v0, v1
+  brff eq v2, block1
+  jump block2
+  ; check:  ucomiss %xmm1, %xmm0
+  ; nextln: jp      label2
+  ; nextln: jnz     label2; j label1
+
+block1:
+  v3 = iconst.i32 1
+  ; check: movl    $$1, %eax
+  return v3
+  ; check: ret
+
+block2:
+  v4 = iconst.i32 2
+  ; check: movl    $$2, %eax
+  return v4
+  ; check: ret
+}