arm_neon.h produces a large number of errors under -fno-lax-vector-conversions,
which disables some deeply dangerous implicit conversions and which we really
need to make the default (PR17164).
This patch fixes the immediate problem by removing a bunch of incorrect casts
generated in the header. But it's only the first part of the problem: the other
part is fixing the incorrect expectations in the corresponding CodeGen test
files (in particular, in two ~20K line test files). However, I don't want to
touch that until I'm confident this change is correct.