Add regspill and regfill instructions.

These are parallels to the existing regmove instruction, but the divert
the value to and from a stack slot.

Like regmove diversions, this is a temporary diversion that must be
local to the EBB.

cranelift/docs/langref.rst

@@ -700,9 +700,12 @@ allocation pass and beyond.
 .. autoinst:: fill
 
 Register values can be temporarily diverted to other registers by the
-:inst:`regmove` instruction.
+:inst:`regmove` instruction, and to and from stack slots by :inst:`regspill`
+and :inst:`regfill`.
 
 .. autoinst:: regmove
+.. autoinst:: regspill
+.. autoinst:: regfill
 
 Vector operations
 -----------------

cranelift/filetests/parser/tiny.cton

@@ -166,14 +166,21 @@ ebb0(v1: i32):
 ; Register diversions.
 ; This test file has no ISA, so we can unly use register unit numbers.
 function %diversion(i32) {
+    ss0 = spill_slot 4
+
 ebb0(v1: i32):
     regmove v1, %10 -> %20
     regmove v1, %20 -> %10
+    regspill v1, %10 -> ss0
+    regfill v1, ss0 -> %10
     return
 }
 ; sameln: function %diversion(i32) native {
-; nextln: ebb0($v1: i32):
+; nextln:     $ss0 = spill_slot 4
+; check: ebb0($v1: i32):
 ; nextln:     regmove $v1, %10 -> %20
 ; nextln:     regmove $v1, %20 -> %10
+; nextln:     regspill $v1, %10 -> $ss0
+; nextln:     regfill $v1, $ss0 -> %10
 ; nextln:     return
 ; nextln: }