[airconv] expect to eliminate bitcast

[3Shain]

This

%cast_to_int4_by_need_ = bitcast <4 x float> %0 to <4 x i32>
%value_to_store_after_swizzle_mask_915 = insertelement <4 x i32> %cast_to_int4_by_need_, i32 1065353216, i64 3
%1 = bitcast <4 x i32> %value_to_store_after_swizzle_mask_915 to <4 x float>

is equivalent to

%1 = insertelement <4 x float> %0, float 1.0, i64 3

Although bitcast shouldn't impact the runtime behavior... just curious why llvm -O2 optimization doesn't cover this

[3Shain]

A workaround: create an "alias" pointer of float4 array while allocating the register file (which is of int4 array), then we can load/store from/to the alias pointer without problem, and seems llvm can still perfectly promote mem2reg optimization. (which makes sense, since llvm has migrated to opaque pointer type (but we are using an older version of llvm, since the AIR generated by metal still contains typed pointers.