Apply suggestions from code review

Author: aviatesk

test/compiler/ssair.jl

@@ -271,7 +271,7 @@ let code = Any[
         GotoIfNot(Argument(2), 4),
         # block 2
         Expr(:call, throw, "potential throw"),
-        ReturnNode(), # unrechable
+        ReturnNode(), # unreachable
         # block 3
         ReturnNode(Argument(3)),
     ]
@@ -295,7 +295,7 @@ let code = Any[
         ReturnNode(Argument(3)),
         # block 3
         Expr(:call, throw, "potential throw"),
-        ReturnNode(), # unrechable
+        ReturnNode(), # unreachable
     ]
     ir = make_ircode(code; slottypes=Any[Any,Bool,Bool])
     visited = BitSet()