[lld] [COFF] Minor tweaks to ARM64 relocation code. NFC.

[mstorsjo]

Summary

Fix issues found in existing code, while reviewing other changes.

Change the data type of a variable to uint32_t, to avoid potential issues with signedness in shifts.

[mstorsjo]

Moving a shift as well.

COFF/Chunks.cpp

 static void applyArm64Imm(uint8_t *Off, uint64_t Imm) {
   uint32_t Orig = read32le(Off);
   Imm += (Orig >> 10) & 0xFFF;
   Orig &= ~(0xFFF << 10);
   write32le(Off, Orig | ((Imm & 0xFFF) << 10));
 }
 
 static void applyArm64Ldr(uint8_t *Off, uint64_t Imm) {
-  int Size = read32le(Off) >> 30;
+  uint32_t Size = read32le(Off) >> 30;
   if ((Imm & ((1 << Size) - 1)) != 0)
     fatal("misaligned ldr/str offset");
-  Imm >>= Size;
-  applyArm64Imm(Off, Imm);
+  applyArm64Imm(Off, Imm >> Size);
 }
 
 void SectionChunk::applyRelARM64(uint8_t *Off, uint16_t Type, OutputSection *OS,
                                  uint64_t S, uint64_t P) const {
   switch (Type) {
   case IMAGE_REL_ARM64_PAGEBASE_REL21: applyArm64Addr(Off, (S >> 12) - (P >> 12)); break;
   case IMAGE_REL_ARM64_PAGEOFFSET_12A: applyArm64Imm(Off, S & 0xfff); break;
   case IMAGE_REL_ARM64_PAGEOFFSET_12L: applyArm64Ldr(Off, S & 0xfff); break;