[DAGCombiner] Support {a|s}ext, {a|z|s}ext load nodes in load combine

Resubmit -r295314 with PowerPC and AMDGPU tests updated.

Support {a|s}ext, {a|z|s}ext load nodes as a part of load combine patters.

Reviewed By: filcab

Differential Revision: https://reviews.llvm.org/D29591

llvm-svn: 295336
diff --git a/llvm/test/CodeGen/ARM/load-combine-big-endian.ll b/llvm/test/CodeGen/ARM/load-combine-big-endian.ll
index 047c732..c88cdb7 100644
--- a/llvm/test/CodeGen/ARM/load-combine-big-endian.ll
+++ b/llvm/test/CodeGen/ARM/load-combine-big-endian.ll
@@ -456,17 +456,12 @@
 ; (i32) p[1] | (sext(p[0] << 16) to i32)
 define i32 @load_i32_by_sext_i16(i32* %arg) {
 ; CHECK-LABEL: load_i32_by_sext_i16:
-; CHECK: ldrh  r1, [r0]
-; CHECK-NEXT: ldrh  r0, [r0, #2]
-; CHECK-NEXT: orr r0, r0, r1, lsl #16
+; CHECK: ldr  r0, [r0]
 ; CHECK-NEXT: mov pc, lr
-
+;
 ; CHECK-ARMv6-LABEL: load_i32_by_sext_i16:
-; CHECK-ARMv6: ldrh  r1, [r0]
-; CHECK-ARMv6-NEXT: ldrh  r0, [r0, #2]
-; CHECK-ARMv6-NEXT: orr r0, r0, r1, lsl #16
+; CHECK-ARMv6: ldr r0, [r0]
 ; CHECK-ARMv6-NEXT: bx  lr
-  
   %tmp = bitcast i32* %arg to i16*
   %tmp1 = load i16, i16* %tmp, align 4
   %tmp2 = sext i16 %tmp1 to i32