Write and parse value locations for EBB arguments

Fixes #56.

We now have complete support for value location annotations in the
textual IL format. Values defined by instructions as well as EBB
arguments are covered.

cranelift/filetests/parser/instruction_encoding.cton

@@ -5,7 +5,7 @@ isa riscv
 ; regex: WS=[ \t]*
 
 function %foo(i32, i32) {
-ebb1(v0: i32, v1: i32):
+ebb1(v0: i32 [%x8], v1: i32):
     [-,-] v2 = iadd v0, v1
     [-] trap
     [R#1234, %x5, %x11] v6, v7 = iadd_cout v2, v0
@@ -14,7 +14,7 @@ ebb1(v0: i32, v1: i32):
     [Iret#5] return v0, v8
 }
 ; sameln: function %foo(i32, i32) native {
-; nextln: $ebb1($v0: i32, $v1: i32):
+; nextln: $ebb1($v0: i32 [%x8], $v1: i32):
 ; nextln:     [-,-]$WS $v2 = iadd $v0, $v1
 ; nextln:     [-]$WS trap
 ; nextln:     [R#1234,%x5,%x11]$WS $v6, $v7 = iadd_cout $v2, $v0