i#1551 Port DynamoRIO to ARM: use "pop pc" for ldm mangling if possible
i#1551 Port DynamoRIO to ARM: use "pop pc" for ldm mangling if possible

When we normalize "pop {rx, pc}", we inserted two instructions at the end:
"add sp, 4; ldr [sp, -4] pc", which is a read-beyond-TOS.
We replace those two instructions with "pop pc" to make it faster and
avoid read-beyond-TOS.

Review-URL: https://codereview.appspot.com/218080043

code4bones/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,bl4ckic3/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,daksunt/dynamorio,sigma-random/dynamorio,code4bones/dynamorio