ARM: fix fast-isel assertion failure

Missing braces on if meant we inserted both ARM and Thumb load for a litpool
entry. This didn't end well.

rdar://problem/15959157

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200752 91177308-0d34-0410-b5e6-96231b3b80d8