Revert of PPC/s390: Internalize strings in-place (reland) (patchset #1 id:1 of...

Revert of PPC/s390: Internalize strings in-place (reland) (patchset #1 id:1 of https://codereview.chromium.org/2628643002/ )

Reason for revert:
Original CL https://codereview.chromium.org/2626893005 got reverted.

Original issue's description:
> 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}
> Committed: https://chromium.googlesource.com/v8/v8/+/aa0cb2eaf830e03fced1e00134fb65938df02388

TBR=jkummerow@chromium.org,joransiu@ca.ibm.com,jyan@ca.ibm.com,michael_dawson@ca.ibm.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:4520

Review-Url: https://codereview.chromium.org/2628923004
Cr-Commit-Position: refs/heads/master@{#42304}

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) == 0xa7);
+                 kShortExternalStringMask) == 0x93);
   __ andi(r4, r3, Operand(kIsNotStringMask | kStringRepresentationMask |
                           kShortExternalStringMask));
   STATIC_ASSERT((kStringTag | kSeqStringTag) == 0);
@@ -1423,9 +1423,9 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ ble(&runtime);
   __ SmiUntag(r4);
 
-  STATIC_ASSERT(8 == kOneByteStringTag);
+  STATIC_ASSERT(4 == kOneByteStringTag);
   STATIC_ASSERT(kTwoByteStringTag == 0);
-  STATIC_ASSERT(kStringEncodingMask == 8);
+  STATIC_ASSERT(kStringEncodingMask == 4);
   __ 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) == 0xa7);
+                 kShortExternalStringMask) == 0x93);
   __ mov(r3, Operand(kIsNotStringMask | kStringRepresentationMask |
                      kShortExternalStringMask));
   __ AndP(r3, r2);
@@ -1421,9 +1421,9 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
   __ ble(&runtime);
   __ SmiUntag(r3);
 
-  STATIC_ASSERT(8 == kOneByteStringTag);
+  STATIC_ASSERT(4 == kOneByteStringTag);
   STATIC_ASSERT(kTwoByteStringTag == 0);
-  STATIC_ASSERT(kStringEncodingMask == 8);
+  STATIC_ASSERT(kStringEncodingMask == 4);
   __ ExtractBitMask(r5, r2, kStringEncodingMask, SetRC);
   __ beq(&encoding_type_UC16, Label::kNear);
   __ LoadP(code,