[PATCH] Modify MIPS long branch for NaCl

[Mark Seaborn]

LGTM

BTW, if you're hitting this when targeting NaCl at the moment, you should probably turn off PIC code generation, since you don't need PIC for NaCl nexes.

================
Comment at: lib/Target/Mips/MipsLongBranch.cpp:474
@@ +473,3 @@
+        // don't know the exact amount of code that "sandboxing" will add, we
+        // conservatively estimate that code will not grow more than 100%.
+        Offset *= 2;
----------------
Could the code grow by more than a factor of 2?  Some explanation of the max expansion factor here would be good.

e.g. "lw $sp, ($reg)" requires 2 sandboxing instructions to be added (before and after).

http://reviews.llvm.org/D3928