S390: Fix prologue offset in GeneratePrologue

Updates the prologue_offset value in LCodeGen::GeneratePrologue after
generating ahi/aghi.
prologue_offset is used in FindCodeAgeSequence. Failing to update
prologue_offset will result in failing to correctly identify the code
aging sequence (young).

R=joransiu@ca.ibm.com, mbrandy@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1843673002

Cr-Commit-Position: refs/heads/master@{#35112}

src/crankshaft/s390/lithium-codegen-s390.cc

@@ -108,7 +108,8 @@ bool LCodeGen::GeneratePrologue() {
     // Prologue logic requires its starting address in ip and the
     // corresponding offset from the function entry.  Need to add
     // 4 bytes for the size of AHI/AGHI that AddP expands into.
-    __ AddP(ip, ip, Operand(prologue_offset + sizeof(FourByteInstr)));
+    prologue_offset += sizeof(FourByteInstr);
+    __ AddP(ip, ip, Operand(prologue_offset));
   }
   info()->set_prologue_offset(prologue_offset);
   if (NeedsEagerFrame()) {