Fix 64-bit copy to SCC

Fix 64-bit copy to SCC by restricting the pattern resulting
in such a copy to subtargets supporting 64-bit scalar compare,
and mapping the copy to S_CMP_LG_U64.

Before introducing the S_CSELECT pattern with explicit SCC
(0045786f), there was no need
for handling 64-bit copy to SCC ($scc = COPY sreg_64).

The proposed handling to read only the low bits was however
based on a false premise that it is only one bit that matters,
while in fact the copy source might be a vector of booleans and
all bits need to be considered.

The practical problem of mapping the 64-bit copy to SCC is that
the natural instruction to use (S_CMP_LG_U64) is not available
on old hardware. Fix it by restricting the problematic pattern
to subtargets supporting the instruction (hasScalarCompareEq64).

Differential Revision: https://reviews.llvm.org/D85207