No need to map over all blocks, setting up PIC.

Darwin x86 has inconsistent PIC base register, so splitting (which happened before)
ensures that each cmm procedure only has one entry point (namely the first block).

compiler/nativeGen/PIC.hs

@@ -771,19 +771,11 @@ initializePicBase_x86 ArchX86 os picReg
              BasicBlock bID (X86.FETCHGOT picReg : insns)
 
 initializePicBase_x86 ArchX86 OSDarwin picReg
-        (CmmProc info lab live (ListGraph blocks) : statics)
-        = return (CmmProc info lab live (ListGraph blocks') : statics)
-
-    where blocks' = case blocks of
-                     [] -> []
-                     (b:bs) -> fetchPC b : map maybeFetchPC bs
-
-          maybeFetchPC b@(BasicBlock bID _)
-            | bID `mapMember` info = fetchPC b
-            | otherwise            = b
+        (CmmProc info lab live (ListGraph (entry:blocks)) : statics)
+        = return (CmmProc info lab live (ListGraph (block':blocks)) : statics)
 
-          fetchPC (BasicBlock bID insns) =
-             BasicBlock bID (X86.FETCHPC picReg : insns)
+    where BasicBlock bID insns = entry
+          block' = BasicBlock bID (X86.FETCHPC picReg : insns)
 
 initializePicBase_x86 _ _ _ _
         = panic "initializePicBase_x86: not needed"