[spv-out] fix countLeadingZeros impl

[teoxoy]

• id should have had the result of the last operation
• MathOp::Custom should have been returned since we later execute: self.cached[expr_handle] = id;
• removed unnecessary bitcast

[evahop]

Sorry about that. This was done in tandem with the glsl backend. Looks like aspects of that ended up over here without too much consideration.

[teoxoy]

No worries, I haven't noticed it either.
SPV validation tests also didn't catch it because we never used the returned value.

[jimblandy]

Yes, looks better now.