[lld][WebAssembly] Fix memory.fill argument in 64-bit mode

[sbc100]

Summary

This only effects folks building with wasm64 + shared memory which
is not currently a supported configuration in emscripten or any other
wasm toolchain.

lld/test/wasm/data-segments.ll

 ; DIS-NEXT:            i32.const       0
 ; DIS-NEXT:            i32.const       20
 ; DIS-NEXT:            memory.init     2, 0
 ; NOPIC-DIS-NEXT:      [[PTR]].const   1064
 ; PIC-DIS-NEXT:        [[PTR]].const   40
 ; PIC-DIS-NEXT:        global.get      1
 ; PIC-DIS-NEXT:        [[PTR]].add
 ; DIS-NEXT:            i32.const       0
-; DIS-NEXT:            i32.const       10000
+; DIS-NEXT:            [[PTR]].const   10000
 ; DIS-NEXT:            memory.fill     0
 
 ; NOPIC-DIS-NEXT:      [[PTR]].const   11064
 ; PIC-DIS-NEXT:        local.get       0
 
 ; DIS-NEXT:            i32.const       2
 ; DIS-NEXT:            i32.atomic.store        0
 

lld/wasm/Writer.cpp

for (const OutputSegment *s : segments) {
           if (config->isPic) {
             writeU8(os, WASM_OPCODE_LOCAL_GET, "local.tee");
             writeUleb128(os, 1, "local 1");
           }
         }
 
         if (s->isBss) {
           writeI32Const(os, 0, "fill value");
-          writeI32Const(os, s->size, "memory region size");
+          writePtrConst(os, s->size, is64, "memory region size");
           writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
           writeUleb128(os, WASM_OPCODE_MEMORY_FILL, "memory.fill");
           writeU8(os, 0, "memory index immediate");
         } else {
           writeI32Const(os, 0, "source segment offset");
           writeI32Const(os, s->size, "memory region size");
           writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
           writeUleb128(os, WASM_OPCODE_MEMORY_INIT, "memory.init");