[ARM] Adjust isLegalT1AddressImmediate for non-legal types

Types such as float and i64's do not have legal loads in Thumb1, but will still
be loaded with a LDR (or potentially multiple LDR's). As such we can treat the
cost of addressing mode calculations the same as an i32 and get some optimisation
benefits.

Differential Revision: https://reviews.llvm.org/D62968

llvm-svn: 362874