Use movw+movt in ARMFastISel::ARMMaterializeGV.

This eliminates a lot of constant pool entries for -O0 builds of code
with many global variable accesses.

This speeds up -O0 codegen of consumer-typeset by 2x because the
constant island pass no longer has to look at thousands of constant pool
entries.

<rdar://problem/10629774>

llvm-svn: 147712