unhappy codegen handling of invalid tuples

[vtjnash]

MWE for a codegen bug observed "in the wild"

julia> f(a, b) = a[1] = b[1]
f (generic function with 1 method)

julia> @code_llvm optimize=false raw=true f(Tuple{Int}[], Tuple{Union{}}[])
julia: /data/vtjnash/julia/src/intrinsics.cpp:287: llvm::Value* emit_unboxed_coercion(jl_codectx_t&, llvm::Type*, llvm::Value*): Assertion `to->isIntOrPtrTy() || to->isFloatingPointTy()' failed.

signal (6): Aborted
in expression starting at REPL[3]:1
gsignal at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
abort at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
unknown function (ip: 0x7fc43c965728)
__assert_fail at /lib/x86_64-linux-gnu/libc.so.6 (unknown line)
emit_unboxed_coercion at /data/vtjnash/julia/src/intrinsics.cpp:287
emit_unbox at /data/vtjnash/julia/src/intrinsics.cpp:353
typed_store at /data/vtjnash/julia/src/cgutils.cpp:1485
emit_builtin_call at /data/vtjnash/julia/src/codegen.cpp:2984
emit_call at /data/vtjnash/julia/src/codegen.cpp:3692
emit_expr at /data/vtjnash/julia/src/codegen.cpp:4509
emit_ssaval_assign at /data/vtjnash/julia/src/codegen.cpp:4160
emit_stmtpos at /data/vtjnash/julia/src/codegen.cpp:4402 [inlined]
emit_function at /data/vtjnash/julia/src/codegen.cpp:7191
jl_emit_code at /data/vtjnash/julia/src/codegen.cpp:7564

https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2021-06/03/SplittablesBase.1.7.0-DEV-9f32653992.log

split from #40065 (comment)

(prior to f313b08, we didn't have an assert here and would just return Undef)