Greedy: Make eviction broken hint cost use CopyCost units (#160084)

Change the eviction advisor heuristic cost based on number of broken
hints to work in units of copy cost, rather than a magic number 1.
The intent is to allow breaking hints for cheap subregisters in favor
of more expensive register tuples.

The llvm.amdgcn.image.dim.gfx90a.ll change shows a simple example of
the case I am attempting to solve. Use of tuples in ABI contexts ends up
looking like this:

  %argN = COPY $vgprN
  %tuple = inst %argN
  $vgpr0 = COPY %tuple.sub0
  $vgpr1 = COPY %tuple.sub1
  $vgpr2 = COPY %tuple.sub2
  $vgpr3 = COPY %tuple.sub3

Since there are physreg copies in the input and output sequence,
both have hints to a physreg. The wider tuple hint on the output
should win though, since this satisfies 4 hints instead of 1.

This is the obvious part of a larger change to better handle
subregister interference with register tuples, and is not sufficient
to handle the original case I am looking at. There are several bugs here
that are proving tricky to untangle. In particular, there is a double
counting bug for all registers with multiple regunits; the cost of
breaking
the interfering hint is added for each interfering virtual register,
which have repeat visits across regunits. Fixing the double counting
badly
regresses a number of RISCV tests, which seem to rely on overestimating 
the cost in tryFindEvictionCandidate to avoid early-exiting the eviction
candidate loop (RISCV is possibly underestimating the copy costs for
vector registers).