Aggressive constprop in norm

[jishnub]

This helps reduce TTFX by not compiling the unused branches.

On nightly,

julia> using LinearAlgebra

julia> A = rand(2,2);

julia> @time norm(A);
  0.445819 seconds (341.96 k allocations: 17.589 MiB, 38.80% gc time, 99.99% compilation time) # v"1.12.0-DEV.558"
  0.162979 seconds (341.23 k allocations: 17.554 MiB, 99.98% compilation time) # This PR