Fix slow compilation of large zero-initialized arrays; warn on large non-zero fills

[39ali]

let arr = [0; 32 * 1024] compiled very slowly. The cause: Rvalue::Repeat with a zero element routes through memset, while will create a huge list of repeated store instructions, the fix: use OpConstantNull instead when fill_byte == 0, valid for any SPIR-V composite type, single instruction regardless of size.

SPIR-V has no equivalent of OpConstantNull for non-zero values , so the best we can do is tell the user early by emitting a warning