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

MIPS32 has instructions for efficient count-leading/trailing-zeros, so this should be
considered a cheap operation (and therefore fair game for speculation) for any MIPS32
implementation.

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

ctlz:
  jr  $ra
  clz  $2, $4

cttz:
  addiu  $1, $4, -1
  not  $2, $4
  and  $1, $2, $1
  clz  $1, $1
  addiu  $2, $zero, 32
  jr  $ra
  subu  $2, $2, $1

Instead of:

ctlz:
  beqz  $4, $BB0_2
  addiu  $2, $zero, 32
  clz  $2, $4
$BB0_2:
  jr  $ra
  nop

cttz:
  beqz  $4, $BB1_2
  addiu  $2, $zero, 32
  addiu  $1, $4, -1
  not  $2, $4
  and  $1, $2, $1
  clz  $1, $1
  addiu  $2, $zero, 32
  subu  $2, $2, $1
$BB1_2:
  jr  $ra
  nop

See D14469 for the larger motivation.

Differential Revision: http://reviews.llvm.org/D14500

llvm-svn: 252755