[ARM] Fix so immediates and pc relative checks

[simonwallis2]

Summary

An SoImm 32 bit immediate constant is an 8 bit value 0 to 255
rotated right by a 4-bit even count, 0, 2, . . 28, 30.

Most negative values that are multiple of 4 in the range -1020 to -4 cannot be represent by an SoImm.

This commit stops considering an SoImm encoding for a negative offset to a constant island.

[simonwallis2]

Abandoning this patch.
I overlooked that NegOk is fine for ADR, encoding A2.
I failed to look beyond encoding A1.

llvm/lib/Target/ARM/ARMConstantIslandPass.cpp

for (MachineInstr &I : MBB) {
           case ARM::LEApcrel:
           case ARM::LEApcrelJT:
             // This takes a SoImm, which is 8 bit immediate rotated. We'll
             // pretend the maximum offset is 255 * 4. Since each instruction
             // 4 byte wide, this is always correct. We'll check for other
             // displacements that fits in a SoImm as well.
             Bits = 8;
             Scale = 4;
-            NegOk = true;
             IsSoImm = true;
             break;
           case ARM::t2LEApcrel:
           case ARM::t2LEApcrelJT:
             Bits = 12;
             NegOk = true;
             break;
           case ARM::tLEApcrel: