[AVR] Add ISel pattern which maches load-from-global, compile it to

LPMRdZ (avr-rust/rust-legacy-fork#47,
avr-rust/rust-legacy-fork#48)

Author: gergoerdi

lib/Target/AVR/AVRInstrInfo.td

@@ -1443,6 +1443,7 @@ def STDWPtrQRr : Pseudo<(outs),
 // Load program memory operations.
 let canFoldAsLoad = 1,
 isReMaterializable = 1,
+mayLoad = 1,
 hasSideEffects = 0 in
 {
   let Defs = [R0],
@@ -1469,8 +1470,7 @@ hasSideEffects = 0 in
                 Requires<[HasLPMX]>;
 
   // Load program memory, while postincrementing the Z register.
-  let mayLoad = 1,
-  Defs = [R31R30] in
+  let Defs = [R31R30] in
   {
     def LPMRdZPi : FLPMX<0,
                          1,
@@ -2033,6 +2033,9 @@ def Asr16 : ShiftPseudo<
 // Non-Instruction Patterns
 //===----------------------------------------------------------------------===//
 
+def : Pat<(load (add i16:$offset, (AVRWrapper tglobaladdr:$base))),
+          (LPMRdZ (SUBIWRdK i16:$offset, tglobaladdr:$base))>;
+
 //:TODO: look in x86InstrCompiler.td for odd encoding trick related to
 // add x, 128 -> sub x, -128. Clang is emitting an eor for this (ldi+eor)