[ARM] Rewrite isLegalT2AddressImmediate

[ARM] Rewrite isLegalT2AddressImmediate

This does two main things, firstly adding some at least basic addressing modes
for i64 types, and secondly treats floats and doubles sensibly when there is no
fpu. The floating point change can help codesize in some cases, especially with
D60294.

Most backends seems to not consider the exact VT in isLegalAddressingMode,
instead switching on type size. That is now what this does when the target does
not have an fpu (as the float data will be loaded using LDR's). i64's currently
use the address range of an LDRD (even though they may be legalised and loaded
with an LDR). This is at least better than marking them all as illegal
addressing modes.

I have not attempted to do much with vectors yet. That will need changing once
MVE is added.

Differential Revision: https://reviews.llvm.org/D60677

llvm-svn: 358845