[AArch64][GISel] Fold COPY(y:gpr, DUP(x:fpr, i)) -> UMOV(y:gpr, x:fpr, i) (#89017)

This patch adds a peephole to AArch64PostSelectOptimize for codegen
that is caused by RegBankSelect limiting G_EXTRACT_VECTOR_ELT
only to FPR registers in both the input and output registers. This can
cause a generation of COPY from FPR to GPR when, for example, the
output register of the G_EXTRACT_VECTOR_ELT is used in a branch
condition.

This was noticed when looking at codegen differences between SDAG and GI
for the s1279 kernel in the TSVC benchmark.