Simplify relocate implementation

We're adding 1 to the high bit of the low12 chunk, letting that carry through
the high bits, and then keeping only the high bits.  In this case, the high
bits are always zero.  Spelling this out since I found it confusing at
first... Given an int12, two possible cases:
val                         hi20     low12
b31-b12 b11 b10..b0
111..11 1   (dont-care)     0        trunc(val)
000..00 0   (dont-care)     0        trunc(val)

Author: preames

lld/ELF/Arch/RISCV.cpp

@@ -469,14 +469,12 @@ void RISCV::relocate(uint8_t *loc, const Relocation &rel, uint64_t val) const {
 
   case INTERNAL_R_RISCV_X0REL_I:
   case INTERNAL_R_RISCV_X0REL_S: {
-    assert(isInt<12>(val));
-    uint64_t hi = (val + 0x800) >> 12;
-    uint64_t lo = val - (hi << 12);
+    checkInt(loc, val, 12, rel);
     uint32_t insn = (read32le(loc) & ~(31 << 15)) | (X_X0 << 15);
     if (rel.type == INTERNAL_R_RISCV_X0REL_I)
-      insn = setLO12_I(insn, lo);
+      insn = setLO12_I(insn, val);
     else
-      insn = setLO12_S(insn, lo);
+      insn = setLO12_S(insn, val);
     write32le(loc, insn);
     return;
   }