[ARM] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

[ARM] add overrides for isCheapToSpeculateCttz() and isCheapToSpeculateCtlz()

ARM V6T2 has instructions for efficient count-leading/trailing-zeros, so this should be
considered a cheap operation (and therefore fair game for speculation) for any ARM V6T2
implementation.

The net result of allowing this speculation for the regression tests in this patch is
that we get this code:

ctlz:
  clz  r0, r0
  bx  lr
cttz:
  rbit  r0, r0
  clz  r0, r0
  bx  lr

Instead of:

ctlz:
  cmp  r0, #0
  moveq  r0, #32
  clzne  r0, r0
  bx  lr
cttz:
  cmp   r0, #0
  moveq  r0, #32
  rbitne  r0, r0
  clzne  r0, r0
  bx  lr

This will help solve a general speculation/despeculation problem noted in PR24818:
https://llvm.org/bugs/show_bug.cgi?id=24818

Differential Revision: http://reviews.llvm.org/D14469

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252639 91177308-0d34-0410-b5e6-96231b3b80d8