[X86] Handle BSF/BSR "zero-input pass through" behaviour (#123623)

Intel docs have been updated to be similar to AMD and now describe
BSF/BSR as not changing the destination register if the input value was
zero, which allows us to support CTTZ/CTLZ zero-input cases by setting
the destination to support a NumBits result (BSR is a bit messy as it
has to be XOR'd to create a CTLZ result). VIA/Zhaoxin x86_64 CPUs have also
been confirmed to match this behaviour.

This patch adjusts the X86ISD::BSF/BSR nodes to take a "pass through"
argument for zero-input cases, by default this is set to UNDEF to match
existing behaviour, but it can be set to a suitable value if supported.

There are still some limits to this - its only supported for x86_64
capable processors (and I've only enabled it for x86_64 codegen), and
Intel CPUs sometimes zero the upper 32-bits of a pass through register
when used for BSR32/BSF32 with a zero source value (i.e. the whole
64bits may not get passed through).

Fixes #122004

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