Dynamic stack realignment use of sp register as source/dest register

Dynamic stack realignment use of sp register as source/dest register
in "bic sp, sp, #15" leads to unpredicatble behaviour in Thumb2 mode.
Emit the following code instead:
mov r4, sp
bic r4, r4, #15
mov sp, r4

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90724 91177308-0d34-0410-b5e6-96231b3b80d8