[SILGen] When emitting an apply, don't decode the dbgloc unconditionally.
[SILGen] When emitting an apply, don't decode the dbgloc unconditionally.

Turns out it's really expensive and not really needed. On my machine
the testcase reported in SR-7691 goes down from 24 seconds to 2 seconds
after this change.

<rdar://problem/40258978>

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