Also consider fixed outputs for replace_global_defines.

Fixes #178.

When an instruction with a fixed output operand defines a globally live
SSA value, we need to check if the fixed register is available in the
`regs.global` set of registers that can be used across EBB boundaries.

If the fixed output register is not available in regs.global, set the
replace_global_defines flag so the output operands are rewritten as
local values.

cranelift/filetests/regalloc/global-fixed.cton

@@ -0,0 +1,17 @@
+test compile
+set is_64bit=1
+isa intel haswell
+
+function %foo() native {
+ebb4:
+    v3 = iconst.i32 0
+    jump ebb3
+
+ebb3:
+    v9 = udiv v3, v3
+    jump ebb1
+
+ebb1:
+    v19 = iadd.i32 v9, v9
+    jump ebb3
+}