[8u] RFR: assembler canonical modified imm encoding

[Anton Kozlov]

Hi, All!

In 'prec1' commit, modified imm encoding was changed to use constant amount of time for any immediate.
Sadly, new encoding became not canonical i.e. rotate value may be not minimal as possible, which is not recommended, but allowed by ARM. Start of -XX:+PrintInterpreter looks like
  0x72df8320: mov       r3, sp
  0x72df8324: sub       sp, sp, #88     ; 0x58
  0x72df8328: str       lr, [sp]
  0x72df832c: mov       r2, r6
  0x72df8330: mov       r1, sl
  0x72df8334: str       r5, [fp, #-32]  ; 0xffffffe0
  0x72df8338: ldr       r9, [fp, #-12]
  0x72df833c: cmp       r9, #0, 8
  0x72df8340: beq       0x72df8374
  0x72df8344: push      {r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, sl, fp, ip, lr}
  0x72df8348: add       ip, sp, #224, 2 ; 0x38
  0x72df834c: push      {ip}            ; (str ip, [sp, #-4]!)
  0x72df8350: movw      r0, #33092      ; 0x8144
  0x72df8354: movt      r0, #30085      ; 0x7585
  0x72df8358: mov       r1, pc
  0x72df835c: sub       r1, r1, #128, 4

Note instructions like 
  0x72df833c: cmp       r9, #0, 8 // zero encoded as 0 rotated at 8.

The patch restores recommended encoding (still constant time encoding), after applying listing looks like
  0x72dae320: mov       r3, sp
  0x72dae324: sub       sp, sp, #88     ; 0x58
  0x72dae328: str       lr, [sp]
  0x72dae32c: mov       r2, r6
  0x72dae330: mov       r1, sl
  0x72dae334: str       r5, [fp, #-32]  ; 0xffffffe0
  0x72dae338: ldr       r9, [fp, #-12]
  0x72dae33c: cmp       r9, #0
  0x72dae340: beq       0x72dae374
  0x72dae344: push      {r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, sl, fp, ip, lr}
  0x72dae348: add       ip, sp, #56     ; 0x38
  0x72dae34c: push      {ip}            ; (str ip, [sp, #-4]!)
  0x72dae350: movw      r0, #45548      ; 0xb1ec
  0x72dae354: movt      r0, #30080      ; 0x7580
  0x72dae358: mov       r1, pc
  0x72dae35c: sub       r1, r1, #8

Sorry for temporary inconvenience in debugging. Happy hacking again!

Thanks,
Anton
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