PPC/s390: Internalize strings in-place (reland)

Port 4c699e34

Original Commit Message:

    using newly introduced ThinStrings, which store a pointer to the actual,
    internalized string they represent.

    (Previously landed as #42168 / af51befe.

R=jkummerow@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:4520
LOG=N

Review-Url: https://codereview.chromium.org/2628643002
Cr-Commit-Position: refs/heads/master@{#42202}

src/ppc/code-stubs-ppc.cc

@@ -1386,7 +1386,7 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   // (1) Sequential string?  If yes, go to (4).
 
   STATIC_ASSERT((kIsNotStringMask | kStringRepresentationMask |
-                 kShortExternalStringMask) == 0x93);
+                 kShortExternalStringMask) == 0xa7);
   __ andi(r4, r3, Operand(kIsNotStringMask | kStringRepresentationMask |
                           kShortExternalStringMask));
   STATIC_ASSERT((kStringTag | kSeqStringTag) == 0);
@@ -1424,9 +1424,9 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ ble(&runtime);
   __ SmiUntag(r4);
 
-  STATIC_ASSERT(4 == kOneByteStringTag);
+  STATIC_ASSERT(8 == kOneByteStringTag);
   STATIC_ASSERT(kTwoByteStringTag == 0);
-  STATIC_ASSERT(kStringEncodingMask == 4);
+  STATIC_ASSERT(kStringEncodingMask == 8);
   __ ExtractBitMask(r6, r3, kStringEncodingMask, SetRC);
   __ beq(&encoding_type_UC16, cr0);
   __ LoadP(code,

src/s390/code-stubs-s390.cc

@@ -1383,7 +1383,7 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   // (1) Sequential string?  If yes, go to (4).
 
   STATIC_ASSERT((kIsNotStringMask | kStringRepresentationMask |
-                 kShortExternalStringMask) == 0x93);
+                 kShortExternalStringMask) == 0xa7);
   __ mov(r3, Operand(kIsNotStringMask | kStringRepresentationMask |
                      kShortExternalStringMask));
   __ AndP(r3, r2);
@@ -1422,9 +1422,9 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ ble(&runtime);
   __ SmiUntag(r3);
 
-  STATIC_ASSERT(4 == kOneByteStringTag);
+  STATIC_ASSERT(8 == kOneByteStringTag);
   STATIC_ASSERT(kTwoByteStringTag == 0);
-  STATIC_ASSERT(kStringEncodingMask == 4);
+  STATIC_ASSERT(kStringEncodingMask == 8);
   __ ExtractBitMask(r5, r2, kStringEncodingMask, SetRC);
   __ beq(&encoding_type_UC16, Label::kNear);
   __ LoadP(code,