[LV] Improve register pressure estimate at high VFs

Previously, `getRegUsageForType` was implemented using
`getTypeLegalizationCost`.  `getRegUsageForType` is used by the loop
vectorizer to estimate the register pressure caused by using a vector
type.  However, `getTypeLegalizationCost` currently only appears to
understand splitting and not scalarization, so significantly
underestimates the register requirements.

Instead, use `getNumRegisters`, which understands when scalarization
can occur (via computeRegisterProperties).

This was discovered while investigating D118979 (Set maximum VF with
shouldMaximizeVectorBandwidth), where under fixed-length 512-bit SVE the
loop vectorizer previously ends up costing an v128i1 as 2 v64i*
registers where it actually occupies 128 i32 registers.

I'm sending this patch early for comment, I'm still doing some sanity checking
with LNT.  I note that getRegisterClassForType appears to return VectorRC even
though the type in question (large vNi1 types) end up occupying scalar
registers. That might be worth fixing too.

Differential Revision: https://reviews.llvm.org/D125918