byte code instructions can fit in a byte

This isn't particularly important.  It just struck me that if we're
going to be byte-thrifty, might as well go all the way.

Change-Id: I4d137731ac06643c50e6bc66cc4654abad732cea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310325
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>

Author: Mike Klein

src/core/SkRuntimeEffect.cpp

@@ -420,7 +420,7 @@ static skvm::Color program_fn(skvm::Builder* p,
             default:
                 #if 0
                     fn.disassemble();
-                    SkDebugf("inst %04x unimplemented\n", inst);
+                    SkDebugf("inst %02x unimplemented\n", inst);
                     __builtin_debugtrap();
                 #endif
                 return {};

src/sksl/SkSLByteCode.cpp

@@ -47,7 +47,7 @@ using U32 = skvx::Vec<VecWidth, uint32_t>;
 
 static const uint8_t* DisassembleInstruction(const uint8_t* ip) {
     auto inst = READ_INST();
-    printf("%04x ", (int)inst);
+    printf("%02x ", (int)inst);
     switch (inst) {
         DISASSEMBLE_COUNT(kAddF, "addf")
         DISASSEMBLE_COUNT(kAddI, "addi")

src/sksl/SkSLByteCode.h

@@ -19,7 +19,7 @@ namespace SkSL {
 class  ExternalValue;
 struct FunctionDeclaration;
 
-enum class ByteCodeInstruction : uint16_t {
+enum class ByteCodeInstruction : uint8_t {
     // B = bool, F = float, I = int, S = signed, U = unsigned
 
     kAddF,  // N

src/sksl/SkSLByteCodeGenerator.cpp

@@ -633,7 +633,7 @@ void ByteCodeGenerator::write(ByteCodeInstruction i, int count) {
         case ByteCodeInstruction::kMaskBlend: this->exitCondition();  break;
         default: /* Do nothing */ break;
     }
-    this->write16((uint16_t)i);
+    this->write8((uint8_t)i);
     fStackCount += StackUsage(i, count);
     fMaxStackCount = std::max(fMaxStackCount, fStackCount);