x86/string compression: Use TESTB instead of TESTL in String.charAt().

And fix disassembly of the now unused TESTL.

Test: testrunner.py --host with string compression enabled.
Test: Manual inspection of dump-oat output.
Bug: 35433135
Bug: 31040547
Change-Id: I36c955bc1f2243954ecc315266a2f3fce5d87693
diff --git a/disassembler/disassembler_x86.cc b/disassembler/disassembler_x86.cc
index ff05733..a289433 100644
--- a/disassembler/disassembler_x86.cc
+++ b/disassembler/disassembler_x86.cc
@@ -1306,7 +1306,7 @@
     has_modrm = true;
     reg_is_opcode = true;
     store = true;
-    immediate_bytes = ((instr[1] & 0x38) == 0) ? 1 : 0;
+    immediate_bytes = ((instr[1] & 0x38) == 0) ? (instr[0] == 0xF7 ? 4 : 1) : 0;
     break;
   case 0xFF:
     {