s390: Refactor atomic ops to distinguish Int32/Int64 ops

Port 3db1d4a5

Original Commit Message:

    Currently, atomic operations assume the default to be 32-bit
    operations, fix opcode names for differentiation between 32/64-bit
    operations.

R=gdeepti@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: I79c195ce7474f329ccb2446fad1f81bab41e7329
Reviewed-on: https://chromium-review.googlesource.com/937921Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#51580}

src/compiler/s390/instruction-selector-s390.cc

@@ -2280,10 +2280,12 @@ void InstructionSelector::VisitWord32AtomicLoad(Node* node) {
   ArchOpcode opcode = kArchNop;
   switch (load_rep.representation()) {
     case MachineRepresentation::kWord8:
-      opcode = load_rep.IsSigned() ? kWord32AtomicLoadInt8 : kAtomicLoadUint8;
+      opcode =
+          load_rep.IsSigned() ? kWord32AtomicLoadInt8 : kWord32AtomicLoadUint8;
       break;
     case MachineRepresentation::kWord16:
-      opcode = load_rep.IsSigned() ? kWord32AtomicLoadInt16 : kAtomicLoadUint16;
+      opcode = load_rep.IsSigned() ? kWord32AtomicLoadInt16
+                                   : kWord32AtomicLoadUint16;
       break;
     case MachineRepresentation::kWord32:
       opcode = kWord32AtomicLoadWord32;