IUPAC bases in bcfnorm are converted to N instead of the bases from the VCF spec

[bartcharbon]

We are using bcftools norm with --check-ref e, but it fails for structural variants in regions with IUPAC bases in the reference genome.
The SV caller we are using converts the IUPAC in the VCF REF field like:

R -> A
Y -> C
S -> C
W -> A
K -> G
M -> A
B -> C
D -> A
H -> A
V -> A

Which I think is following the latest VCF specs.
But since bcftools norm converts the non-N IUPACS to N the comparison then fails.